I got the twitches for these too, so here's some recent builds, as I failed to even wait "IM"-patiently for the Racer releases to start rumbling our way, I decided to add to the trifecta of Racing Capris I own.
I started with the Revell BMW 320i, which is still front motored, and with original mechanics, but sitting on Slot.it F22 F1 size tyres which are 13mm wide, heavily ground down to a suitable O.D. Everything is glued and trued, and the motor has been swapped for a Slot.it yellow bell, and geared down somewhat.
Build thread here http://slotcarillustrated.com/portal/forums/showthread.php?t=61646
Having since done two more cars, I think I'll revisit this one and lower it another 1mm It looks too high against everything else.
Then I moved to a Fly Porsche 935, which proved a challenge. The motor in those sits BEHIND the rear axle, so I had to do away with that, the wheels were at about a 45 degree angle to the axle - well, not quite, but very bad.
Build thread here - it was really quite simple
sheesh, thank goodness Wet Coast Racer introduced me to JB Weld.
In the end, I glued in a Slot.it inline pod, with a 1/8th brass rod either side of it, running from the front to rear axles, to stiffen the stupidly flexible chassis. and used a Ninco NC-5 motor which is about the same grunt as the Fly FK180 motors in the Fly Racing Capri. It is sitting on Slot.it 17 x 10 magnesium wheels, the tyres are Slot.it N22 from memory. I don't have a suitable wheel insert yet, so any ideas very welcome.
Lastly I tackled the Fly BMW 635 CSL, which is also front motored. I have again swapped the motor for a Slot.it yellow bell. Like the 320, I managed to keep the whole drive train. When I get some brass tube to shim them, the stub front axles need their pins collared to remove the slop. The rest of the bushes got the superglue collar trick, and I fitted NSR 21 x 14mm Vanquish MG tyres to the original ubs, which were "fairly" straight - given that it is a FLY. They were a decent fit but needed about 1mm off the O.D., and the outside edges well rounded. Then they fitted well under the body.
I sectioned the interior glass, forced the roll cage to slew back about 3mm so it sits in the top of the roof where the clear "glass" was removed. I also dremeled the underside of the cockpit fairly brutally, and heated the front tab and melted it so it sat flatter against the underside of the bonnet. This allowed me to move the front of it upwards 1.5mm into the body, which then allowed me to shorten the body posts 1mm to lower the bodyand still clear the chassis with the cockpit.
The most radical change was slicing the sides of the chassis, and gluing them to the underside of the body, as the centre part of the chassis was "clam-shell" style which is not ideal for wood running. I first attached them by melting the attaching tabs with the soldering iron, then laid a layer of JB weld against the insides, and a layer of thin wire mesh against it and forced it hard, so JB weld spewed through the mesh, then smoothed it all off. They'll never split in a shunt now.
The chassis can now float free of the body. Once the drive train had had the slop taken out of the driveshaft bush and axles bushes, the "grating" of badly meshing Fly gears pretty much disappeared.
It is running pretty smooth, but getting the fronts more stable will help eventual cornering speeds.
They've been trued down, the exhaust pipes moved so they clear the body - by a fraction, and the track, but not much either. Overall ride height is at least 1mm lower than stock. It looks the absolute business running at low elevation angle.
GR. 5 are some of the most brutal "bad attitude" race cars you'll ever see on a track.