Ok, I'll assume if you can pass a pilots certificate test, you are plenty smart to handle RC lipo without burning the house down. You don't need to buy a new bike, you just need to modify it.
Start reading about this chemistry, often bought from Hobby King in china. It has a c rate much greater than the typical for lifepo4 bike batteries. This allows the use of any hot rod controller you like, and as a bonus, it's smaller and lighter too.
18s would be a good voltage, 63v minimum at the end of the ride, That would get your motor to go close to 50kph at the very end, and likely 55 kph as you leave the garage. Most "72v" controllers have a low voltage cutoff at about 60v, so just about perfect for a drop dead stopping point for 18s lipo. 15s could work too, depending on the exact winding your motor has. That should get you about 40-45 kph, using the controller you have now.
Backing up a bit, you might try dipping your toe in lipo by adding a 3s lipo pack in series to your existing batteries. See if that is fast enough. If so, then you could leave your bike stock, and just add 12v more for a small increase in speed.
Hauling ass, you will never get good wh/mi. Period. So just accept that it will take a fairly big battery to go 20k at 50kph. 40 wh/mi is not that unusual, for a bike that goes fast. 18s lipo in 15 ah size should do the trick. 18s 15 ah would be 9 of the 6s 5000 mha packs. 3 sets of 3 packs in series, each set made from 3 6s packs connected parallel.
BTW, I have a LTA certificate, hot air. If a dumbass balloon pilot can figure out lipo, surely a fixed wing pilot can. Like aviation, it's dangerous, but safe if you just follow established safe routines.