My next project bike is going to be a two stroke powered looney. I was thinking of getting a aluminium frame from a 125 sport bike and fit a 250 liquid cooled two stroke parallel twin into it. After much searching I came up with nothing as selling prices for the frames are pie in the sky prices and that was just for the bare frame and so the idea died away. Then while thinking what else is there I can do that will confuse people when they see it and sound totally unlike it should and have unexpected performance.. So with cost being a major factor I have decided to track down a Hyosung GTR 125 or 250 and sell on the engine. Very early days but should be interesting. Oh the engine choice is a MK1 Suzuki 250 gamma engine due to it not having a power valve so will be raw power delivery.
A Megelli frame should also be cheap. They have full fairing model with good brakes and reasonable susension.
I had similar idea for track bike. Wanted responsive geometry and tight wheelbase ~1350mm. Suddenly NSR250 MC21 frame popped up!! I'm building it up with old used RZ350 engine first. Good test-bed for dialing in Microsquirt. Hopefully it'll last one season. While I build up 421cc Banshee engine with TURBO!! Similar to this one.
How about KTM RC390 frame? I was looking at those originally. Really light and stiff! Very common here since so many of those engines blew up. Are you handy with welder? Can build your own trellis frame.