As you may or may not know, FHI makes engines for a variety of machines in addition to Subaru vehicles, including Polaris snowmobiles and various small Robin generators....
But that is a pretty danged cool toy. Especially if you could do a few things to it... Forced induction... higher output sparks... Or put it on something different like a little peddle car.
No, really you'd just want to send the compression to high heaven and run that nitro stuff they use in r/c cars. Maybe convert it to water cooled. Mount the radiator up front with a baffle to make sure air runs through it. Have the water pump run off the chain somewhere. People would laugh at you until you go flying past them on a really hot muggy day and all their little air-cooled bikes are bogging down.