Originally Posted by jrecoi

I'm tending to agree with Lenton Sports. The shifter and cable routing is clearly an aftermarket affair, from the 60s-70s, and possibly the rear wheel is also aftermarket. The shifter pulley is still there; normally these are fitted close to the top tube, in conjunction with a fulcrum clip on the top tube. The current arrangement is more typical of the 70s.

We need more information on the stampings on the tire and rim, if it is 26 x 1 1/4 or 26 x 1 3/8, this can help or hinder finding additional tires. 26 x 1 1/4 rims are rather hard to find tires for, at least in the US (and impossible to find in Colombia), although it doesn't hurt to poke around in Buenos Aires. 26 x 1 3/8 tires are much easier to find in the US, I don't doubt that in a capital city like Buenos Aires, you can find tires for this size, particularly in the older, more established bike shops.

The stem looks like it has a crack at the handlebar clamp, you might want to check the diameter of the handlebar, Sheldon Brown warned that stuffing a 25.4mm handlebar in the 23.8mm handlebar clamp can cause the kind of crack seen here. This is a bit of a mixed blessing, on one hand, the stem may or may not be fatally stuffed, on the other hand, this frees you to buy a classic looking stem that fits currently available handlebars.