What’s the Impedance I need?

I have this 1968 Univox E3 (or 1235) that I’ve had for at least 15 years or so. I sold the cab ages ago, and I’d like to revive this amp and give it a second shot at life as a guitar amp. Trouble is- I don’t know what impedance I need on a new cab to function correctly. Anyone here know what it might be, or how I’d find out?

Question 2: How hard is it to replace the old 2-prong cord to a safer one with a ground?

 

Nobody?

 

Perfect. Thank you!

It will be seeing a tech before it’s recommissioning for sure.