The front-motor in-line arrangement seems at first to be an unlikely drive train configuration for a slot car, since it places the motor's weight at the opposite end of the car from where it's needed for maximum cornering grip.  However, with the introduction of strong neodymium traction magnets it has been quite successful on cars for plastic sectional track with steel contact strips.  Fly has used the arrangement in its Viper, Panoz, Marcos, Corvette C5R, C5, BMW 3.5 csl, BMW- M3 BMW-320i, Ferrari-365 GTB4, Lister-Storm, Alfa 147 GTR and  Ford Capri,  Scalextric uses it in its Lotus and Caterham Sevens, and Corvette L-88 and TVR Tuscan 400R. Also on the Monogram Shelby Cobra Daytona, Monogram Corvette Grand Sport. MRRC's Cobra motor layout. 

All the Fly front-motor cars use exactly the same motor and drive shaft assembly  with the exception of the Marcos, which differs only in having a shorter drive shaft length. The motor, shaft bushing, pinion gear, and spring coupler are the same on all of them, as are the various mounting points in the chassis. (Drawing 1) Thus, if you have any Fly motor and drive shaft assembly you can use it in any of the cars or in a scratch built or kit bashed car using a modified Fly chassis simply by lengthening or cutting the shaft if necessary.  Since the shaft diameter on all Fly cars and on every motor currently being used in 1:32 scale home racing cars is .078? You can easily make a longer shaft by getting a piece of .078? steel wire from the K&S metal rack at your local hobby shop and cutting a length to fit your car.  You can use any end bell-drive Mabuchi S can or other motors with the same external configuration in a Fly front-motor car.  The motor will fit the chassis and the drive shaft and spring coupler will mate up with the motor's shaft.  In some cases, however, you may need to trim the motor shaft, either on the end that mates with the drive shaft or on the opposite end to clear parts of the chassis.  This can be done easily with a Dremel Moto-tool equipped with a carbide cutoff wheel.  Be sure not to let the motor shaft get too hot when cutting it, as this can damage the bushings,

The MRRC Cobra uses a virtually identical shaft assembly that can be interchanged with Fly's except for shaft length.  MRRC sells the shaft assembly without a motor, so if you need driveline components but don't need to change the motor get the MRRC parts and make a longer shaft from steel wire if you need to.  Also, since neither MRRC nor Fly sells the shaft bushing as a separate part this is the least expensive way to get one.  These come two to a bag, and if you do motor swaps or other work that involves R&Ring the drive shaft it's a good idea to keep a couple of them handy.

Removing the drive shaft from the motor and reconnecting it without ruining the coupler spring is not difficult, but many people have trouble getting the spring off without stretching it.  The key is always to push the spring off the motor shaft or the drive shaft, as shown in Drawing 2.  Place the jaws of your needle-nose pliers on the motor shaft between the end bell bushing and the spring coupler. Slide them along the shaft, pushing the spring coupler as you go.  The coupler will normally come right off with only light pressure.  Use the same technique to remove the coupler from the drive shaft as shown in Drawing 3.

When it's time to put the coupler spring back in place it just pushes onto the shaft.  Fly motors have a splined shaft end to better grip the spring coupler.  If you are installing a hopup motor, such as a Slot It V12, it won't have the splined shaft end and may not fit the coupler tightly.  In that case, use a drop of medium CA glue in the spring coupler to hold it tightly to the shaft (Drawing 4).  Don't use the thin CA, as it can easily run into the shaft bushing and lock up the motor.  Before using any kind of CA on a motor shaft or a drive shaft be sure to put a drop of oil in the motor and drive shaft bushings.  Be careful not to get any oil where the CA glue will go.

You can use a gear puller to remove the pinion gear from a drive shaft assembly without removing the shaft from the motor, but you have to remove the shaft before you can press a pinion back on.  Once the spring coupler is removed you can use a pinion press on the drive shaft exactly as if it had a motor around it.  If you do a lot of gear changes you will save time by making up several shafts, each with a pinion gear with a different number of teeth.  Whenever you put the shaft assembly back together don't forget to put the shaft bushing back on before reattaching the drive shaft to the motor shaft.

One of the biggest problems with Fly front-motor cars is the drive shaft bushing's tendency to pop out of its mounting in a crash or even a hard spin.  When this happens the pinion gear starts chewing teeth off the crown gear and after that the car never has a smooth, quiet gear mesh.  We have heard from customers who have glued or wired the bushing in place, but we don't recommend either of those measures because they make the bushing difficult to remove when you need to.  There's a better way, as shown if Drawing 5.  All you have to do is cut a piece of styrene strip or balsa about 1/8? by 1/4? and glue the end of it to the bottom of the car's interior tray or tub so the end of it presses down on the shaft bushing when the body is in place on the chassis.  It will take a bit of cutting and trying to get the exact location and length needed, but once you get it right it will permanently end all problems with the shaft bushing coming loose without making it hard to remove. Also use hot glue from a hot glue gun works well too and is not permanent