you will be lacking topspeed, unless you do some modifications. You can get some more out of them, but you need to work on the manifold and pipe. The general problem with offroad engines is that they are not made for such long full throttle, high RPM operation. They tend to overheat, and once they start overheating, temp goes up real quick.

I have been racing both standard as well as modded buggy and truggy engines. You can compete with them, but you need to be careful with carburation, and if there are pure GT engines in the field, you will see that they spool up just differente from your engine. On a tight track though you might be ok...