Way back in dinosaur times when I was a kid and WWII was happening parts and tires were non existent so shops would grind a crank till it was round and then pour Babbit till the hole in the rod could made to be the same size as the crank. The engines were very low compression/rpm and the roads of that time set an automatic low speed limit. Don't know why a smaller crank pin and bearing would be a problem. The smaller the journal the slower the surface speed and less heat from friction. I'd give one a try in an instant. Would be interesting to set a crank in V-blocks and have a hydraulic ram with a pressure gauge read out to see at what point of under size journals the crank becomes more flexible.

Evan