I would not take the risk.
I agree with Beemerphile.....
I just put a new AGM battery in my ST1300. It is 11AH and works just fine. A 19AH battery has almost twice the stored energy and would quickly start the bike with no problem.
BMS is important for longer life but one could get away with out one. My flashlight, camera, phone, radios, all use Lithium batteries with out BMS balancing. However these devices turn themselves OFF before running the battery into the death zone. Your bike will not do that.
So the part of BMS that you need is to 'disconnect' the battery for undercharge and to prevent overcharging.
The real danger is an accidental short in your wiring that will turn the battery into a road flare / smoke bomb.
I have burned up a few lithium batteries (in a Lab) and I know it would make a real mess of your bike.
The saved weight does not make a difference on a big bike.
Again, I would not take the risk, they do not make ST1300s anymore........