The "motive" SLA are usually only suitable for Electric motors and separate charger, not the charging in a Motor bike. They are different design to the SLA used in Burgler Alarms and Emergency lights etc.
There are actually 3 kinds of SLA, 2 x gel and 1 x mat and as best as I can figure both the Gel kind and Mat Kind originated in 1932 for portable valve radios! By 1937 all new design portables used sealed gel cells in a plastic case, but likely different Gel and Plastic to that used today.
My 1929 McMichael Suitcase has acid burns on inside of case as it used a conventional glass cased "Wet" type lead acid pretty much like one cell of a 1950s motorbike battery (which is 3 cells).
The "Wet" type cells are still best performance & life which is why they are still used for cars, trucks and road-bikes (Racing bikes may be more exotic on batteries).
Both 6V Bike and even 6V car batteries are still made and available.