I finally found time to make shortened cranks and install them on the bent, readjust the boom, lengthen the chain and change the front derailleur cable. I ended up reusing an old pair of wide 175mm Suntour MTB cranks: I drilled 13mm holes (9/16x20tpi == 13.02mm inner bore. How lucky ), threaded them with standard R and L pedal hole taps from Park Tool, then faced the outer side of the holes with a 19mm milling bit and filed the inner side flat.

The new holes are 13.5mm deep, as opposed to 15mm for the 175mm original holes, due to the fancy triangular section I had to mill through to create a flat spot, but the pedals don't screw all the way through anyway, so I reckon they'll be just fine.

Here's what the cranks look like: