Hi
As Rohan states Mike Pemberton & Pushrod Performance seem to know what he is doing.
Big problem with the Jawa it is not a Norton, other issue is to address the total loss oiling system?
My 500DT-89 Jawa engine cost less than $695 brand May 1976, (noticed one on Allen Ebay linkt for $1,500??) and replaced it for very much worked B33, there was no comparison for power and reliability.
Big ends were an issue and needed some work after around 12 meetings, rings also needed changing, valve springs, yes you could do a years racing with only top end work, if you didn't attend to them you just stopped running up the front.
Had an early iron head ES2 in a Aerial Huntmaster frame, high compression piston, hot cams and ported/reangled inlet and bigger carby, that also went very quick in the late sixties.
Regards
Burgs