It doesn't have to be much of a heatsink. Any heatsink will do, and even just bolting it onto the face of an aluminum L-channel should suffice for keeping it at a reasonable and safe temperature.
The Trim pin is used with an external circuit to adjust the voltage of the outputs. I can't remember what Sync is for. Neither of them are particularly useful for our needs because none of the items we would be hooking this up to will require voltage accuracy.
I have a very similar one, but...
The plethora of screws holding the battery compartment closed could be replaced or supplemented with quick-release pins. But I would also recommend adding a hinge to it near the neck.
TNCscooters has one for $10
http://tncscooters.com/product.php?sku=103250
,but it has a 4-hole pattern and is intended to be compatible with the freewheel they sell.
http://tncscooters.com/product.php?sku=103210
Since we're all having fun photoshopping your project, here's my stupid idea. Can the suspension pivot support a tool steel drive axle? Or is the bearing too small for that?
If it could, then you could mount the motor on a bracket attached to the swing arm linkage. You could then use a 1:1 chain...
If the bolt pattern for the torque arm could do without just one or two of the bolts without compromising strength, you could have those drilled all the way through at a much smaller size so that you could tap the ends for fittings that would allow you to watercool the hollow axle (if only a...