This may be an unpopular view and I mean no disrespect, but you may well be "fixing" the wrong end of the problem as the 212 is simply an ill-suited engine for retrofitting to a bicycle, it's primary attraction it's low price... initially.
While reliable with a growing aftermarket parts supply, it's also bulky with a poor power to weight ratio, an auxiliary engine with a limited RPM range it's inexpensiveness often negated by the lack of any sort of powertrain, a costly, complicated endeavor to address in most cases, which is what inspired this thread I suspect.
While I advocate "to each their own" & build what you will, if your primary intent is to build affordable & reliable four stroke motorized bicycles, even fast ones with gears you may wish to consider researching other engine & transmission options rather then attempting to redesign a bicycle's derailleur system, it's limitations well known and well short of your needs.
Just for one example, as length & breadth are apparently of little concern as you're a fabricator (I've checked your build, nice custom BTW) this engine type while initially twice the cost of the 212 utility come complete with your choice of transmission options, 3, 4 & 5sps, auto, semiauto & standard - your choice & because of that often cost less, sometimes much less than a completed utility motor build and range from 49cc on up, they've a far better power to weight/displacement to HP ratio as well as full charging systems, electric starters and a vast selection of aftermarket goodies: http://motorbicycling.com/showthread.php?t=19630
Please don't take this the wrong way, the pred is responsible for many a unique build and it's not a bad engine - it just bothers me as it seems many see the displacement vs cost and think that's all there is to it, then get trapped into trying to reinvent the whee... erm, transmission or forced into buying expensive aftermarket custom parts to make it work, pulleys & plates that cost twice as much or more then the engine itself...
If you're a fan of the 212, don't want to buy expensive solutions & not worried about the above, I'd skip the derailleur redesign (they're not all that great in even people powered bicycles lol) & stick w/the sheave drive you've already built, which looks to be a good design & application BTW;
edit: As for the suitability of a cassette/derailleur transmission for motorized application, while the freewheel puts up with it and the normal wear is negligible I can with no more then one missed shift (accidental neutral drop), blow all the teeth off even a quality cassette's cog even with just the stock 49cc four stroke referenced above... granted that's abuse inadvertent or otherwise, but also bodes poorly for it's ability to handle a 212.