Pre 1950s rides of LFGSS: old bikes, vintage rats, classic lightweights

Cotter Pins

The cotters have to fit into the cranks - your milremo cranks must be drilled for the smaller continental 9mm size. There's no problem using them with an English spindle (axle).

There's no reason not to reuse the old ones. If they are marked where they've been up against the flat of the spindle it's worth filing them just to remove the burrs and get them flat again.

I oil the cotter pins when I fit them. As described elsewhere here support the crank so that when you hammer the pins in (not too violently!) you don't damage the BB bearing. After fitting ride a few miles and then give a few light taps on the pins and retighten the nuts. Should be fine after that, although you could check the nuts for tightness after, say, a hundred or so miles.

Bearings

I can't see any advantage in using cages, except it makes assembly easier. But you're not a production line so time saving isn't critical. My method is to put enough 'medium' grease into the cups to hold the balls in place, add some oil (engine oil is OK, but gearbox oil is probably better). Grease the bearing faces on the spindle. Put the balls in the fixed cup (already in the frame) with the frame on lying on its right hand side and insert the spindle making sure you have the longer end of the spindle on the right. You can now use the spindle to hold the balls in place while you screw in the left hand cup which you've already greased, oiled and filled with balls. Adjust and recheck after about a hundred miles.

Gearing

Most people would find 48x18 (72" on 27's) too high, especially with short cranks.

A 19 sprocket is the popular choice - possibly no longer available as a single free. There are plenty of 19 fixed sprockets.

48 x 20 - a bit low for most. 50 x 20 (67" with 27's) is nearly the same as 48x19.

Good luck - I hope you do many miles on this bike.