Fiat 850 Spider
https://www.hemmings.com/magazine/hs...0/3692341.html
I've seen them in decent condition for <$2k. Stock they had an 817cc gas engine making 52bhp and weigh 1600lbs.
But honestly, maybe just try and shoehorn it into a Beetle? They're cheap, common, and the chassis is oh-so-hackable.
Sam