Two buttons does not make a bridge. Of course you can wire the bridge rectifier to only go through one diode, or two in parallel but then with an unregulated design, could be the peak charge voltage goes higher than ideal so could start to boil off battery electrolyte if end of charge is not monitored. Depends on the design.
A 50A diode is going to have a higher, short duration peak current rating, should be fine as long as you're sensible at not cranking it all day long which would burn out the starter too, but it's not hard to find larger diodes at electronics suppliers... just have to heatsink them if you want them capable of anywhere near their rating.
Normally you can just mount them near where the button was, or if you need to add an extra piece of plate aluminum to spread the heat, that's do-able too. Extended charging at 10A is really more of a scenario where your heatsinking needs to keep up, not so much a few seconds cranking.