tsanc - It's not that this thread is only about Losi products, it's just that I'm most familiar with them and probably more qualified to answer a specific question about those cars versus others that I have no experience with.
With that said, I'm not really sure what setups racers have run in the past on the Sportwerks buggies. I know that people were racing them back east with good success, I guess it will just be a matter of asking around. Sorry I'm not much help.
Don - See you in the morning?
r/c newbie - I think those engines are pretty standard in terms of mounting and crank shaft, which means that anything side-exhaust and rotary carb should drop right in. To put the 2.5 or 2.5R in your truck in place of the Pro .15, you will, at the least, need a new header and linkage kit.
killer89 - Funny how you signed your post "killer", haha. Like Don said, you'll just need to tear down and rebuild more often. Gas cars go through a lot more abuse and as such, you need to pay special attention to high wear items. Those rear diff inserts can get sloppy and you'll need to replace them.
Check the diff inserts, dogbone pins, outdrives, gears, etc. and everything you'd look for on your electric cars. One gallon is 30 tanks of fuel...your car could have an average of 15 HOURS of run time through it. Time for a rebuild!