any relatively easy power mods for 18hp vanguard?

[gotmuddy]

anything bolt on? Maybe a custom exhaust?

[oldstreetracer]

Remove the restrictors from the exhaust manifolds. Then get a carbide burr with a long shaft and port match the intake and exhaust ports using the gaskets as a guide. Then smooth the port runners so there are no casting ridges etc. If you have small sanding rolls like dremel ones, you can polish the runners. Usually good for a couple of ponies. I do this to all my hot rod motors.