Some things you'll want to determine and list for yourself:
What do you want it to do for you, at what speed, under what terrain and riding conditions?
How far does it have to go under those conditions / speed?
How much are you willing to spend on it?
Once you know the speed, conditions, etc., you can go to the ebikes.ca motor simulator and use it to make a guesstimate of power requirements. It doesn't really matter what parts you choose in the simulator as long as they will supply the power you need to do the job specified by the conditions (weight, slope, etc). Best guess you can simulate using the Cromotor hubmotor listed there, and the phaserunner (if it's not enough use a custom controller with few hundred A limit), and at least a 72v pack.
Once you have figured out the watts it will take to do the job, you can size the motor you need for it. It will also give an estimate of wh/mile, which you can use to determine wh required for hte range you need. Always overestimate, so you always have more watts and more wh than you need.
For Wh, I'd add at least 25-50% capacity to account for detours, headwinds, pack aging, etc, so you always have at least the minimum required range. If you find you need more range than you can fit on the bike, you can install onboard chargers.
If you use a middrive motor (like QS120, 180, etc, or various others) you can keep your CVT/etc, just replacing the ICE with electric. It might require some DIY of building mounting brackets, etc. A midmotor, geared properly, is likely to be smaller and lighter than a hubmotor for the same power output.
If you use a hubmotor, it eliminates the above, but adds DIY torque plates / clamping dropouts / etc to ensure the axle can't spinout from torque (either drive or regen braking). It also opens up more space in the frame for battery...but it adds weight at the wheel which degrades the suspension performance if that's a factor.
For battery, if you are willing to do a lot of reading so you can then DIY one, you can get used EV cells from places like batteryhookup and the like, which are likely to be better than most of the completed batteries you can buy "new". (better matched, better suited to run an EV, etc., likely to perform better overall for longer). It's a fair bit of reading and doing, but for many it is worth it.
Otherwise, you can research the various companies that sell completed packs, and pick the best (probably one of the more expensive ones, but cost doesn't guarantee quality) suited for your usage and space/weight constraints.
I also recommend looking at the various other motorcycle conversion / build threads, as some of them detail out the above stuff a fair bit and may help you with guesstimates for your usage.
