Hi Phil,
Welcome to the best bike on the planet!
Surf over to www.tygaperformance.com for a replacement fairing. Be prepared to empty your wallet into their coffers - they've some very cool NSR gear.
What year is the bike? And do you know which MC21 it is? The R is pretty basic, the SE has twiddly bits on the suspension, the SP has that, a dry clutch and magnesium alloy wheels.
140psi is an excellent compression figure, so it sounds like someone did a rebuild in the not too distant past.
To be honest, the engine behavior doesn't sound too odd to me. All two strokes are a bit 'female' as the power valve does it's thing. What colour are the spark plugs? It may be that you need to alter the jetting a bit to get the mid-range sorted, so the first thing is to get the main jet set right, then start mucking about with needles.
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MC21SP Plaything
BMW F800GS Bumblebee
Triumph 9551 Daytona Big boys toy
FJ1100 Sporting relic
GTS1000 oddball