As mentioned above, finding the exact replacement parts you need might not be the easiest thing...

Bottom bracket cups shouldn't be a problem. Well, except maybe removing the old fixed cup if necessary. Sometimes those can be a bear.

Spindle availability ain't what it used to be since cartridge bottom brackets have been the norm for a few decades. The problem is likely even more acute for cottered spindles. You'll need to get a spindle that's the right overall length for your crankset, the right length between the races, and the right diameter. These types of measurements became more standardized over time, particularly when square taper cranksets ruled the earth, but there were some subtle variations for cottered cranksets.

Since cotter removal is often a destructive process, especially if you don't have access to a cotter press, you may need to source new cotters. You'll want to match the diameter exactly (it varied, largely by country) and find ones whose angle is a close match for your originals. There was a lot of variation, and it was pretty standard practice to file the flats on cotters to get 'em right for a particular crankset.

Personally, on my '72 Peugeot with a cottered crankset, I switched to square taper to avoid a lot of this fuss. I'm still using an older crankset and a traditional adjustable bottom bracket instead of a cartridge to maintain vintage-appropriate appearance, but I have no regrets about ditching cotters.