The battery I replaced was itself a generic replacement and not original from the factory. But after 5-6 years it gave out and so I wanted an upgrade and, not being financially constrained, went looking. I installed an Optima ordered mail order and a yellow dog mounting kit likewise mail order in 20-25 minutes discussed at the end of
this page. But the same process less the special mounting plate would apply for a generic battery and P-car batteries are just batteries. The table in that blog post is really old but the basic notions are the same, any good generic battery of the right size and terminal orientation will work.
It ain't rocket science if I can do it.