Generally, once the keyway gets chewed up most people (and a lot of machinists) will tell you the crank is trash. It can often be welded and a new keyway cut but it's debatable whether it's worth it... that's all up to you.
It's definitely not one of those things where you can just replace the key and maybe squeeze in a little JB Weld.
Can't comment on the overheating, not enough information.
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