g'day mazza. someone correct me if im wrong but i do believe they are using the crf50 plastics.
with regards to thumpstar your right, they have changed their name to terramoto due to another company selling a bike called a thumpsta...
some other bikes you may want to look into would be like the ones that mjc85 posted up, they seem like a nice soild ride, good forks, good shock, very good frame and a decent price. also look into the FMR outlaw, stock 140cc (not a bored 125cc) and a whole bunch of other trick bits. look into the pitster x3 / x3r range as well as these are running a gpx motor (alot of honda inovation has gone into these motors) and are a really nice ride, also the motovert is a nice setup and some at a real nice price. all of these have the crf50 plastics and can take alot of honda parts (cams, pistons, etc etc...)
have a look into these bikes man and let us know what you come up with. you could go with a crf50 too if you wanted to but do consider that they are exceptionally small, slow from the crate, 3 gears semi automatic etc etc...... alot needs to be done and alot needs to be spend on these to get them going as hard as any of the others mentioned.
once again have a look around and let us know what you find out.
Cheers, Tony.