a thumpstar is a type of pit bike and its' seat is about 72cm from the ground
about 70mph
There are a number of places one can find more information on the 125cc Thumpstar. Some of the best sites to find information include eBay, Thumper Talk, Dirt Bike World and Pit Bike Club.
150f is the trail bike where 150r is the motorcross. Trail bike is a good solid bike, as long as you use it for what it's intended, trails. I'm sure it can do some motorocross stuff. If your looking to get into motorcross though, the 150r is better suited.
Thumpstar pit bikes are not easy to find in Canada. XP Power Sports carries them though. Sometimes used ones can be found on Kijiji or ebay (Canada).
honda is a better motorcross bike (250 and 450) and yamaha is a better trail bike
it depends on if you have it licensed and have insurance on it.
maybe 60mph
You do not have to 'get it' or 'sign up' for motorcross. This is something you do on your own, you just train on your own, and of course you have to have a bike and gear, but you just go to the race and register. That's about it. lol
about 50-55 mph
buy a bike some gear and check online for local tracks
After defeating Leon, you take what you want to combine to the dirt bike in his trailor and combine it there.