You can get almost any generic 4X20 scope that has the correct look and it will pass, but the modern ones tend to be about 1/2" shorter than the vintage ASI, Hunter, etc. You will then also need to find the correct mounting clamps. The windage knobs are also incorrect on the newer scopes, but they are passable. I have replaced the entire knob section with a resin cast in the past and that works but then the scope has no adjustment so it doesnt work properly, if that matters to you.
I believe sidewinder includes a replica scope in his EE-3 kits, i think its an aluminum tube and resin cast ends and knob section, but not sure if he'd sell one separately, you'd have to contact him to see.
Ive used these in builds in the past - search e-bay for : Hammers AIR GUN RIFLE SCOPE 4X20 with Dovetail Ring NEW item#171128300341
What ive done with these is unscrew the end with the ribbed ring up until i start seeing the threads, and then just lock it there (i sometimes will glue it as well) so you'll get a touch more length out of it.
If you type in "4x20 rifle scope on e-bay, plenty come up, most are useable, but will need the mounting clamps removed and you will need to find the correct ones. I believe Stormrider and sidewinder both have resin casts they can sell if you dont NEED the real deal.