You should get 30mph out of your current set up, easily more with the upgrades you're talking about and the 26x22 gearing you're probably running at the moment - the numbers are stamped on Tomos gears - try cleaning some of the old grease off if you can't see 'em. The carb and pipe will be a good start.
There are Tomos riders here that can break 40mph with 50cc cylinders - running custom air filters, higher gear ratios and serious performance. I have my own doubts about how long it takes them to get up to that speed, however ;)
...but don't forget it's a 50cc piston-port engine with fixed gearing - you won't get a top speed of 45mph AND hill-climbing performance. There's always a tradeoff between speed and torque, RPM power-band and take-off acceleration. But it's mopeds - experiment! It's easy and cheap to play around, and quite hard to totally screw up - plus if you search the forums and wiki there's plenty of knowledge hanging around to help you out.