OS 91 4st one of my favorites I have 5 of em, 150$ for a older fs not a great deal, you can often pick up a older surpass for 120-150$ of of ebay that will give you more power for your buck, some even new in the box.
the surpass II style of engine is by far the cleanest of the surpass engines since it recurculates the breather oil into the intake manifold, where as with the older surpass has a case vent that a small piece of tubing vents it to the bottom of the cowl or fuse.
tuning is relitively easy, with either the older sp or spII the II being the newer and simplest but there both pretty strait forward, low end screw in the barrel and a high speed needle on the other side.
after buying a couple of the older engines which ran quite well, the carbs were a little sluggish in the tuning dept. and needed cleaning and in one case new rubber seals for better tuning qualities.
they don't call em surpass engines for nothing they have 25-30% more power than the older fs engines and parts are still available finding replacement parts for the older OS engines can be really hit and miss somtimes going to ebay for "parts engines" to canibalize for needed parts.