The frame behind the rear shock could be changed easily - but you are likely going to have to use their fuel tank.....there  is almost nothing that can be done easily with that area.  I guess we are in a similar situation as the folks who worked on cars when the frame went away and cars became "uni-body".  All the new bikes have great engines and handle well - not so much for looks if you don't like the sport bike or transformer image.

Perhaps you need to look for a dual sport chassis.....they may be a bit easier to work with - or just work on older (and cheaper) bikes.

Anything behind the top of rear shock mount should be a bolt on section for ease of maintenance!  It nothing a sawsall can't fix !

Well ok, go ahead ! 😁🏍