If I were in your position I would look on the Electric Motorsport website. They have details of the EMC-RT200 motor from Mars. It is capable of 200Amps continuous which would be plenty for your conversion. The best news is that the cost is only $650. Electric Motorsport can supply a kit with controller, wiring, contactor, charger DC-DC converter, fuses etc.
I'm sure that you could source the parts cheaper, but I got everything except the battery side of things from them so that it all came as one package. It is easier to import this way. You only need to pay one payment for import duty. This part is not difficult at all, so please don't feel that you need to buy from a European based supplier.
There are other US based companies who could supply a similar package, so it may be worth looking around on the internet.
EMS now supply Lithium batteries also, which they did not yet have 2 years ago when I was buying, so you could always Email them or even phone them with your requirements.
I would imagine that you are going to need about 200Wh/mile of energy. That means you will need about 10KWh of battery to have a maximum theoretical range of 50 miles. Of course you would only ever use 80% max. of this storage. Still you might be able to do 40 miles.
I am not thinking of upgrading to Litium but if I was I would be prepared to invest a lot in a BMS to protect my investment in batteries. (The thread on this subject in the batteries section is one of the hottest topics of debate!)
$4000 may be a liitle tight to get everything you need.
I think I said above that I would be happy to use direct drive because of your low speed requirement. Transmissions are heavy. I use a 530 motorcycle chain with sengine sprocket from EMS (12 tooth) and a specially made wheel sprocket (46 tooth) for my 24 inch Kawasaki rear wheel. The EMS website will tell you how many rpm per volt you will get. I calculated 60mph @ 72V. This figure was avery good guess, since a full throttle I get voltage drop., hence only 56mph.
My lighter trike uses about 140Wh/mile unless you go very slowly. That is a figure based on experience, not conjecture, so I hope it is helpful.
Keep us informed about your project. I love reverse trike and think your Isetta is a very good choice for conversion. A Mescerschmitt (think my spelling is wrong!) might be even better because you have the rear seat area as an ideal battery compartment, though you would maybe need a ventilated battery box in there.
Andrew.