Basically a 951 is maxed out from the factory. You can add performance parts like porting and higher compression but your reliability and engine life hours go down dramatically. There were very few pipes made for them so it would be difficult to find one and the twin pipes cracked all the time.
If you wanted to spend some coin sending it to Group K for a sleeper kit wouldn’t be bad.
The stock impeller is actually very good.
And finally if you are rebuilding it you better be doing the crank and balancer too.