It will be great for pre90. I would hang onto it unless you get good dollars for it.
Best engine for pre85 is the GR. No doubt that it is as quick as most heavily modded XSs. It has all the goodies built into it, 270 crank, dual mass flywheel...think like a slipper clutch, splash cooled pistons and 15kg lighter. They just need some development but that is coming. (900cc is not out of the question) Think a lazy 80HP.