Do Chris Robertsons 'variator' mod... it will gain you 3 to 5mph (because of gearing).
Casey has the Proma exhaust and the variator mod and he says his does 42 or so... he says his is fast as heck.
Search for ... PA50 variator mod.
(you make 3 slots in the outer housing of the front pulley.. it takes 10 minutes to get it off... and 15 minutes to make the cuts... be careful... don't make the cuts too deep)
Chris R posted about 4 pics of how to do it.