Tune: That's an awesome looking Ossa. I ran an SDR (Six Day Replica) for a while and it was a lot of fun. My preference is small bikes. I'm not as tall as you so large bikes are more work to ride and just to move around.
Ply, 90 - 100 rear wheel HP needs a spacer plate and a lot of porting including making the transfers wider. At that level, the chassis is just a big heavy noodle and needs help. Forks are too skinny, swingarm is too short and too heavy and not stiff enough. You can fit Mikes XS emulators and SV650 calipers and modern disks and that makes the front end respectable. Get a GS750/100/1100 swingarm to lighten and stiffen the rear end.
Or slip it into an FZR1000 frame with engine cradle cut from the stock frame. That way you can go straight to modern (lighter/stiffer) suspension and wheels. An early FZR frame is really heavy compared to the latest frames, but so much stiffer than a stocker.
I have two mono shock GT projects in the shop that keep getting pushed further back as others take precedence. That's a good way to go too, but GSXR forks are short and if you aren't careful the steering geometry will easily get to be unstable.