Welcome to the forums!
Definitely one awesome
frame IMHO. I agree with 2door
in that the location of the bottom bracket is a hurdle, a glaring one at that. It really needs to be relocated if you plan on an in-frame gas engine. Heck, I cringe looking at the BB for just pedaling, I know I'd tweak that with my half-ton pushing legs. You could easily put a rackmount engine on that bike, but well, we all know that isn't doing that bike justice. Any in-frame job is going to take custom mounts on that animal!
OTOH, you can always go electric! That bike looks to me like the perfect candidate for that. Lots and lots of space. Regardless, a badboy bike like that aint going to be motored cheap