Welcome to the site. Lots of great info and people on here. Dig around, use the search feature and enjoy.
Parts should still be pretty easy to find. There are may sites that still carry a decent amount of oem and aftermarket parts. Ebay is a good place for used and new parts. OEM parts I like to get from
www.bikebandit.com Shipping is reasonable and most of the time the cheapest prices as far as parts go.
As for other engines just bolting in...A 89-94 kdx200 may fit pretty close and maybe a early 90's kx250.