That does look like a nice clutch system there, I can't comment on the quality because I haven't used one. The only thing that could be an issue is the size of the fly wheel If it is too big you can just dremmel out the slot in the chassis so that it will fit, nothing major.
Almost, .21 engines can rev higher than a larger engine. Meaning higher top speed when geared correctly. A larger engine will give you more torque. This will get you to the top speed quicker. The truggy weighs quite a big so the extra torque is needed to get it off the line. Using a .21 will put quite a bit of strain on the conrod. Have a look at good quality .28 engines.