The easiest way is pipe and re-jet, but since you already have this done I would go with an ignition advancer and K&N air filter. You can go with an overbore, but that makes the cylinder walls really thin. There are plenty of aftermarket bits to lighten the bike, bodywork being the best.
Also changing your gearing ratio will help a lot if your looking for acceleration. I did a 520 conversion and went -1 on the front and +2 on the rear. It helped a lot. I wouldn't recommend going any smaller than -1 on the front sprocket and don't do this without going larger on the rear. You will have tons of issues with your chain hitting the chain guide. You will lose some top end, but I can still hit 155-160 with that sprocket setup.
If you have done all of this and still have the need for more speed, an L model 7R crank will drop right in. Because of the L model's longer stroke, you will need a thicker head gasket to accomodate this, but I have heard it changes the engine displacement to around 790cc's. If that isn't enough a B model 9R engine will drop right into the frame. There is a few things you need for the conversion, but all in all it's a pretty simple swap. This is something I have wanted to do for a long time, but whenever an engine pops up on ebay it seems like I never have the money. **** ebay!!!!
If you still need more ideas go to the 7R website in my siggy. Search the site and you will be amazed at what you find for the 7R. Good luck.
