On short tracks an offroad engine is a good option, tons of bottom torque but not that much top RPM. On larger tracks that is a disadvantage of more than a seccond a lap. If you dare to modify it you must change some things to get it more like an onroad engine but then again looking at Novarossi you will see the different conrod can be an early breaking point.
$500 is indeed much. over here there are clubs and a competition running 5 port onroad engines. The N21-5T is an awesome engine and the same is with the Sonic 5 port Basic engine. You will not be disapointed with these.