Four months later:
Alright, so I figured I'd post an update after the 4 month mark... First of all, I've found that the solution for my engine dying is to add an entire bottle of carb cleaner to each gallon of gas (this does nothing for my top-end speed problems though). I've been using 'Super Tech' brand 'Fuel Injector and Carburetor Cleaner' from Walmart, which comes in 8oz bottles marketed as treating "up to 20 gallons"...
Second, my engine has been mounted (from the onset) at a pretty far backwards angle, which definitely helps blow oil into the filter -- I wouldn't be surprised if the carb is full of the stuff too.
So my current thoughts are to get a new engine this spring, and build a new mount with a bit more forwardsy orientation pretty soon too (possibly out of stainless)...
Cheers,
Wolfe.