" Was also wondering about the pinion gear for the clutch since the teeth on it are shredded to bits! Is that a common occurrence with these 66cc motors? "
Not sure what part you're talking about here. If you mean the gears on the right side of the engine I'd have to ask if you have lubricated them. It only takes a dab of grease spread around a little. Too much and it will end up in the clutch.
If there is wear of the gear teeth check to see how they mesh with the secondary gear. Watch for wobbles or an 'off center' condition of the small gear.
If you can run without a tensioner, do it. If you have horizontal drop outs and you have chain to frame clearance you don't need the tensioner. There is also the option of shimming the engine mounts for chain tension adjustment if the bike does not have horizontal drop outs.
There are many pages of discussion here about the infamous 'tensioner' and ways to secure it or options. Try this > http://www.google.com/cse?cx=partner...ner&gsc.page=1