Paul, Check the primary structures, fuselage tubing, wing spars/ribs, for corrosion and damage. I'd have to assume the fabric is not cotton? Remember, paint don't make it fly.
If the primary structures are good, (If you see a lot of rust, bent tubes, splices, walk away), and you can get it for under 30, that is nearing salvage price for a Super Cub. If the O290 checks out, and you think you can fly it for a few years while you save your penneys, a O320 conversion will be about 20K, recover/rebuild another 15to20k.
So... If you can get the plane for about 25 and it is flyable for a few years, then you spend 20 on the O320 conversion, and a while later spend 20 on the rebuild, you have 65K spent on a freshly rebuilt PA18-150, which is not out of line.
Of course, it is safer to spend the 65 now and buy a good PA18-150, but IF the primary structures are good, you may be able to spread the cost out over time, and eventually build the exact plane you want. Good luck.