Go to a motorcycle scrap yard, buy a cheap aircooled twin / triple / four as cheap as possible, completely strip down and rebuild. Preferably blueprint and polish, refurb all parts.
Motorcycle engines are small enough not to need a lot of space or hoists etc.
Next stage build a lightweight single seater chassis to take your engine.
EDIT:
Having seen an ariel arrow twin 250cc modified to a triple 375cc by bolting an additional half engine on and modifying the crank and case and also a friend many years ago turning two 600cc M20 BSA singles into a 1200cc twin (albeit a slowish sidevalve one), that is the sort of project I like. Would like to take a modern motorcycle engine and SLI (
) it.
Motorcycle engines are small enough not to need a lot of space or hoists etc.
Next stage build a lightweight single seater chassis to take your engine.

EDIT:
Having seen an ariel arrow twin 250cc modified to a triple 375cc by bolting an additional half engine on and modifying the crank and case and also a friend many years ago turning two 600cc M20 BSA singles into a 1200cc twin (albeit a slowish sidevalve one), that is the sort of project I like. Would like to take a modern motorcycle engine and SLI (

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