I wonder if it would run on butane? The bottles are smaller and of lighter construction.

I converted a Briggs a while back. It gave 5700 rpm with a 20x8 running on gas. You are not far off with the propane. I also did a poulan 4 stroke. It gave 5400 with the 20x8. It is much lighter than the Briggs. I saw the Lehr brand propane weedwackers on clearance about a year ago. I almost bought one to mess with.

I would probably make an aluminum refillable tank. I can't advise that due to possible bursing hazards, just saying what I would do.