You mean compatibility with parts from non-Recon kits? Your best bet would be to start with RC18 stuff. Here are the parts I tried:
TheToyz RC18 Steel diff gear & input gear - bevel gear fits perfectly, centre hole on the diff gear needs to be reamed slightly larger but works great after that.
3Racing RC18 Heavy duty diff tube - won't work, too wide. First you need to widen the lower bulkhead by 0.5mm or so. After that, the dogbones are pushed too far out and you can't connect the upper camber links any more. Possibly solved with using shorter dogbones, but I don't know where you can find something exactly the right length.
Suspension arms, uprights, knuckles and C-hubs might work. By eye, it looks like the bulkheads, upper deck, and shock towers are too different. I'd love to get a better steering track but again these look different too. Don't know if the RC18 slipper clutch will fit but I'd like to go for that if there's anyone else who can confirm.
These are not really chassis-specific, but I have swapped out all my electronics.
ESC: Turnigy Trackstar 18A (same as SkyRC Toro 25A, but the Turnigy programming card is easier to get)
Motor: generic 5300kV 2030 motor from Hobbyking, needs centre hole in motor mount reamed, new mounting holes drilled, and screws
Servo: Corona CS-929MG, soon to be DS-929MG.
Rx: Spektrum SR300
I love these little Corona servos, the 919/929/939 are all excellent performers for just a few dollars each.