What matters is what your bike outputs now.Would it be because the rectifier/regulator? The owner before us replaced the old regulator/rectifier with a new one. I'm wondering how much AC does the stator/alternator outputs for 1978 GS750?
I'd stick my dual trace oscilloscope on the alternator outputs and know in about 3 seconds, not sure how you can do it.
Seriously why do you want a lithium battery, because everybody and their cousin tries to put one on their project?