Rise from the depths!

Motorcycle Experts! I SUMMON THEE!

So I'm undertaking a little project, I guess you could call it a go-kart of sorts, except it's larger. My idea is to build a small car in the style of a Bugatti Type 35/1920s racecar, only instead a single-seater, and slightly longer than an Austin Mini. I envision the thing to weigh between 300 and 400 pounds.

I've gone through a bunch of car engines measurable in the ccs, but nothing fits. I surmise my only decent option is a bike engine. Long story short: are there any motorcycle engines which I can use, which are fairly reliable/durable, and which aren't too fast or powerful (I don't want to kill myself)?

Your help is greatly appreciated! :)