Yeah I love the engine in my 620 S it is a fun ride for sure. It is actually going to become my track bike now that I have the ms4. Good suggested by triduc on the gearing. I think most people drop a tooth on the front sprocket (15t to 14t). You might also want to look at some swapping the 750 forks for some adjustable SBK ones. Not a power upgrade, but will give you some adjustablity of the handling. Also you should look at cutting the airbox top (or some aftermarket airbox mod) and getting an aftermarket exhaust to open her up a little bit. The intake sounds awesome on the 2v IMO. You will have to rejet the carbs though, which it might need anyway.

You will also want to check out another active forum member's site (Monster Man) at http://www.ducatisuite.com/ Great list of tools needed. And also a list of mods he did to his 98 m750 that would probably interest you.