That's right. Originals were 1.35V and they were mercury oxide, long since banned. A long time ago I did find a source of Panasonic 1.35V that were alkyline, but they've dried up too (pun intended).
As much as I want originality, there's just no other choice today but to convert to the solid state set-up.
I think there are 2 ways it's done. The tach head can be converted, which then operates like a Sun Super Tach, and the sender becomes a dummy, nothing more than a junction box for wiring (sits on the inner fender for looks). Or the sender itself can be converted to no longer need batteries, but then it needs an extra wire run to it which hurts the original appearance.
Or.....you "can" use AAs, and the needle will move, but you won't like what it does.
Verne