I have two bikes with cottered cranks and I think I will face the cotterpin reinstallation issue fairly soon.
It occurs to me that with a tall stack of washers, or a piece of pipe tubing of the right diameter that's fairly thick, plus a hobby vise or unmounted vise, you could probably create a makeshift cotterpin press.
You just need to stack the washers or pipe on the crank arm around the cotter pin threads, and then attach the vise to the crank arm. Make sure to use some sheet-copper or brass to shield the cotter and/or pipe from scratching. Put one jaw on the cotter head, and the other jaw on the washers or pipe. Then, crank the vise closed to press in the (greased) pin.
This depends upon being able to find washers or pipe with an inside diameter of 10mm+, and a fairly narrow outside diameter so as not to hit the crankarm spider.
Best wishes and good luck,
- Don Gillies
San Diego, CA, USA