Yeah, I love light bikes and 2-strokes in particular. Got started riding as kid on 2-stroke dirt-bikes well before I got licence.
I've got one of these rollers built up from pile of parts without engine:
https://japan.webike.net/HONDA/NSR25.../m-spec/y1991/
About 280-lbs with double Ninjette power. But as you mentioned, engine & parts are rare. So I went with modern RD/RZ350 engine out of Banshee. HUGE aftermarket support, can even build entire engine completely from aftermarket parts! Power starts @ ~50hp and up to 100hp with bore & stroke. Then add boost....
Trick to emissions with 2-stroke is EFI! Time injection just before transfer port closes and you have very little sneaking out exhaust port. KTM and Husky has some nice 2-stroke dirt-bikes with EFi that passes Euro-5 emissions requirements. Another trick is timed oil-injection as well:
https://www.ninjette.org/forums/showthread.php?t=379627