Track down the Unertl scope mount for it, and an old rimfire scope...
The 513T is becoming collectible, and prices are climbing. If you list one in as good a condition as yours is in the US for $500, it will sell, fast... They don't seem to go for as much up here, because they aren't as well known.
My uncle has one, with the Unertl mount and an old 4x rimfire scope (smaller than 1 inch ring, can't remember what the exact diameter is), and the thing is frighteningly accurate, with pretty much any ammo you run through them.
These were originally created for marksmanship training for the US military in WWII, so they could train ROTC and DCM on less expensive ammo. If it has "US Property" marked on the barrel and receiver, it's a serious collector's piece.
Even without the "US Property" markings on it, they're still fairly collectible. And they're still used on competition fairly frequently down in the states. One of the most accurate mass production .22 rifles ever made.
Try Numrich Gun Parts Corp (in the US) to see if they have the proper scope mount for it. You'll be able to find one of the older smaller diameter rimfire scopes for it without trouble at pretty much any gun show for less than $100 (probably less than $50, actually, if you really hound the tables at a gun show).