[markosjal]

I found a John A English locomotive for sale. I assume it is old it is die cast metal apparently and works.

How old would this beast be?
What is the coupler?
How hard would it be to put an NEM pocket or Kaydee coupler on it?

 

[Eilif]

I'm pretty sure you're looking at mid 50's.
You can see a fair bit if info about English/Hobbyline (Same company) here;
https://hoseeker.com/hobbyline.html
They've got diagrams for your loco as well and it appears in a couple catalogs between 55 and 57. Can't quite tell from the pics what kind of coupler it had.