If you still have your betamax player in a working condition, connect it to any dvd- or hard disk recorder that can record video via the output of that player (probably plain old analog coax antenna signal, maybe scart also). Then press play on the player and record on the recorder.

It's as simple as that.

A PC with TV card and TV recording software should work also. But there's nothing wrong in this case with using dedicated hardware.

Kees