These are really good questions, and I wish I had a specific answer, but I haven't machined some of those grades and I'm a little confused about the properties you want.
If "everybody" is getting away with 12L14, at least in normal flight, they must not need a lot of strength or hardness. They'll get decent toughness with that grade, at the expense of a low yield point. In other words, it may not break in a crash, but it will bend, right?
It sounds like you want more strength and toughness. If so, 4140 is a good choice. Unlike 12L14, adding the "L" in 41L40 doesn't result in a radically easier-to-machine steel. It gives about 25% better machineability than 4140, which is mostly of interest in production screw-machine operations.4340 is better for toughness and strength. These 4000-series alloys get more hardenability than the 40 points of carbon would suggest, because the chromium content aids hardening. And those two are tough steels, reasonable to machine in the normalized condition (which is a way they're commonly sold), and slow-quenching for safe hardening. 4340 is basically oil-hardening -- almost air-hardening in small thicknesses.
If it were me, I'd go for 4340. If 12L14 is hard and strong enough in flight, then 4340 normalized (around 90 ksi yield, IIRC) is a lot stronger than you need for normal flight, and it's very tough. I'd think it would be a good choice to tolerate modest crashes. If you leave it normalized, you wouldn't even have to heat-treat. If you have a fairly rigid lathe, it's a little slow but not difficult to machine.
This is a semi-educated guess, but I think the ultimate material for small-engine cranks might be 8620. It's a low-carbon alloy steel that's really tough. It's made to be case-hardened. If you aren't going to case-harden, then I'd revert to 4340.
If you can get a piece of 4340 normalized to try, and if you find that you can machine it Ok, then you might be set. It's a common grade.
Another thing I know little about is the surface finish you can get, machining small pieces of these various alloys. 12L14 can produce a nice finish even for hackers. With 4340, it could be more difficult, especially if you use the annealed grade. I'd consider finishing the crank and bearing journal(s) by lapping, anyway.