Hi and
to the forum.
It should be possible to mount the eight speed freewheel
(not cassette) to the MP4 but you may need to add a spacer washer
(or two) to the axle to give adequate clearance between the end of the freewheel and the frame, and you may need to spread the frame slightly to accommodate the wider axle
(spreading and aluminium frame is not recommended as it can lead to failure of the frame).
I would go for a seven speed freewheel and some new indexed thumb shifters as I consider them to be the best option with a twist throttle.
I have some shifters similar to yours on one of my bikes and they are a real pain when used in conjunction with an electric motor. If you start off in a low gear and accelerate quickly you cannot operate the trigger action change gear quick enough to keep up with the motor's acceleration! If you are able to remove the existing lever and fit the additional GM brake lever it might be OK, but I suspect that the brake operation may be hindered by the gear levers
(and vice versa).
This is my preferred set-up as I find the twist action throttle control to be more natural and easier to apply smaller throttle movements:
The lever action on the gear shifters makes it very easy to quickly go through the entire range of gears if you suddenly have to stop on a steep hill you can change down to your lowest gear in one swift action ready to pull away quickly. Once moving again you can quickly shift from your lowest gear to the highest gear in one swift action too.
Check out
this thread for more information.
Alan