Sounds like you have a good one. The fast and easy check is if you can fit a quarter between the cylinder head and case it may be a 46.
What prop are you running? We have found the APC 10.5 X 4.5 to be a good prop others have used the APC10 X 4 & 10 X 5 and got good results recently some of us have been running the APC 9.25 X 6 it will turn 17,000 + and scream It wont pull as hard off the start and you have to fly smothe, but it is fast.
Good luck and keep us informed about your races
If you can try and make it to the first ever Club 40 National races to be held October 13th & 14th 2012
www.club40nats.com