Savage_Greg wrote on 12/31/69 at 16:00:05:
It appears to be a stock tank (see the speedo?) with a lengthened tail piece bonded on.  The fat rear tire looks to be automotive -- flat tread rather than rounded to the sidewalls.  Better traction flat but would screw up cornering.  Looks like the saddle is further aft for someone who stands taller than average.

Looking at the convergence of the drive belt, it looks to be attached to something about 9 inches back from the Savage's output shaft -- allowing a stretching of the wheelbase.  I would have gone with a chain conversion since is it nothing to make up a chain to the exact length needed.

And maybe put a turbocharger in the extra space.