I'm looking into the 1/8th on-road class for next year but i can't figure out what motor i need.

Are all 3.5cc motors roughly the same? Are some ported differently for off road maybe?
I was looking at O.S and FX Engines but most come with 180 degree exhaust flanges which i assume are for off road?

If anyone can point me in the right direction or suggest me a good starter motor please let me know.