RE: FS-90
Your FS-91 Surpass (there was no FS-90 Surpass) is from the first series.
Low compression can be caused by a bad piston/ring/liner fit. You may only need a new ring, or any/all of the three. The piston, ring, and liner together will cost about $100.00.
Low compression could also be cause by a bad valve fit. We clean the cylinder head thoroughly and use a bit of toothpaste to lap the valves. If that doesn't work, you may need new valves or a new head with valves. Head with valves is about $120.00, valves only are $70.00 for the pair (with springs and retainers). Valves only will cost about $10.00 less than the full assembies.
If we do the rebuild, add about $30.00 in labor or so, plus postage.
A new FS-91 Surpass II w/o pump runs about $280-$300 from many retailers.