Reeves Custom 50PS non-polarized electrolytic replacement?

[Helmholtz]

While it's not unlikely that the cap developed enough leakage to overheat and eventually blow, there could still be some other amp malfunction involved.

[R.G.]

While it's not unlikely that the cap developed enough leakage to overheat and eventually blow, there could still be some other amp malfunction involved.

Yes. And as a group, we're all over that.
In parallel, the OP may be desperate to get it running again. If that is the case, there is at least something he can do that is not out-and-out crazy to do.
What really needs to happen here, if we were in a perfect world, is for the OP to (1) trace enough of the circuit to tell if there's enough impedance in front of the cap to eliminate causes other than the cap just failing, and to (2) use an oscilloscope to verify that it's not oscillating. Failing those, a fix-it-and-hope strategy might be needed. That's what I was alluding to.

[Helmholtz]

Yes, replacing the NP cap by two anti-series polarized caps with reverse diodes and then testing the amp is a good proposal.
Should be two 10µ/160V caps.

I just wanted to encourage the OP to find out more about the circuit wiring in case there are other problems.