All competition engines come with a rear exhaust. What 1/8 onroad you want to drive? GT or the pancar type?
Differences offroad/GT and onroad is the size of the cooling head, most offroad/GT are about 60mm diameter and the rotation of the carburettor while with the pancar type the heads are 50-54mm and a switched direction of the carburettor. And yes, technically there are also differences making the needed performance, also with the used exhaust systems (offroad has a longer manifold)

If it is about a GT you basically van use any offroad and GT type engine, if you want to go racing there is a (stupid) limit of 5 ports you need to take care of. Most pancar type engines today have 7 or 9 ports. If you want to start with a decent engine the OS R21max art.nr. OS1B900 is a good start.