I am not even close to an expert on this-hopefully they'll showup momentarily-but I have a few questions:
I don't under stand '3 wheeled EV motorcycle married to a quad w/metro car parts'(i'm easily confused...). Since you want to run the rear wheel i'm assuming you aren't using the Metro's axle (as it has Front-wheel drive). Are you building a Three-wheeled Tadpole custom platform from Quad parts and adapting a Metro chassis to it? Or are you just picking up some choice Metro bits such as wheels, suspension etc to your unique EV frame?
As far as a transmission goes-what do you estimate for the weight of your finished product, and what top speed and climbing abilities do you want? I've seen EVs on this Forum and others successfully use-fixed gear, motorcycle trannies, twin motors with a series/parallel switch, manual trans both with and without clutch, modified Powerglide trans, snowmobile and scooter CVTs, and Deraileurs (see the Bikes section for those

). Once you get the weight/torque/speed questions answered, the Gurus on this Forum should be able to zero in on several good solutions...
BTW, if I were in possession of a decent Metro shell-
(and money, and time, and SKILL, etc etc)


- i'd be tempted to trade out the back axle for a single (unpowered and suspended) back wheel, toss the back seat, keep the manual trans minus clutch and pop in some Lithiums and a good Forklift motor. A Metro Trike running FWD at half the normal weight would be a peppy, decent-ranged EV with the surefootedness of a Mountain Goat...
Good luck with the project BTW!