More pics of the power head would maybe help.
As you says the timing plate can be adjusted, your engine is from the old 35, 45, 55Hp family.
1975 or 1976 Chrysler came with a 35Hp which was from the 25, 30Hp family and at those engines the timing was fixed.
BUT I have never seen a 35Hp from the old family with battery ignition.
They always had magneto from the late 60.es and up.