The SH .21 should fit on the stock chassis, if that is your question. I believe you will need new motor mounts, and all the .21 SH motors I've seen are rear exhaust, so you will need a new header and possibly a new pipe. Also, that motor is a slide carb, so you will need to find some clever way to adapt your servo linkage to open the carb. Finally, I don't know if your flywheel/clutch parts will fit on the .21. I'd assume they would, but if someone more knowledgeable than myself could confirm, that'd be great.
So short answer: yes it will work, but it will take some mods. You can get some ideas here:
www.youtube.com/watch?v=zsvoJEa0GxU
However, the aluminum suspension arms are somewhat (ok, more than somewhat) counter-intuitive. They are expensive, they bend, and even if they hold they just transfer the force of impact to the next weakest piece. I'd rather spend 10 minutes and $3 and change an A-Arm, than either have a bent aluminum arm or a broken diff casing. Other than that, that video seems to have some solid mods.
HobbyHot seems to have the best price on SH engines of anyone out there, although they don't currently have the .21 in stock. The tuned pipe for the .21 engine and the engine itself are here (sorry, somewhat hyperlink retarded at the moment, I managed to switch to the bbcode editor and I don't like it):
http://www.hobbyhot.com/HSP-Model-08...pe-Header.html
http://www.hobbyhot.com/SH-ENGINES-M...ck-Truggy.html
Good luck,
Kristoffer