ash211,
Batteryjunction.com sells 'CR123a size dummy cell' units, as does Lighthound.com (although they cost more), if you don't want to buy a new body. You might want to use some sort of wrap or tubing, if you don't want those rattling around.
The M61LL runs for 10 hours regulated, on 2xCR123a primary cells, and - I think - a good 18650 is going to be very close to that (everyone feel free to jump on that if I've got that wrong), so, theoretically, you might get close to 20 hours on 2x18650. YMMV.
Something else to keep in mind: 3xCR123a is the length of 2xAA (roughly), so 3x18650 is roughly the length of 4xAA. That voltage range is going to work with any AA NiMH, alkaline (in an emergency), lithium primary, or NiZN (1.7 volts?) rechargeable. Just don't pop in any of the new batteryjunction.com 3 volt lithium primaries. You will want something to keep them from rattling around. I would suggest wandering the aisles of a good hardware store. Maybe plastic pipe, maybe vinyl tubing. You might need some sort of fibre washer inside the lens, to push the led module back, to help with connectivity, or not. I definitely need that sort of arrangement with a 9P & A14, when using 3xAA cells, but you have a different set-up. If the 4xAA length is right, buy yourself two CR123a dummy inert spacer cells, and you've got yourself a fairly versatile set up.
I don't have the thread reference handy, but someone ran a runtime test with the older Malkoff M60LL, and it was running (out-of-regulation) at 5 lumens after 80 hours (3+ days!), on 3xAA. It was not as bright as the full output at the beginning, because of the voltage, but ran longer. The increased voltage of 4xAA might or might not make it run harder, in regulation, with decreased runtime, but you could always get a dummy inert spacer AA unit (lighthound has them), and hopefully repeat the same results with the M61LL. The versatility of the M60/M61LL circuitry is phenomenal, the lack of a focused throwing-beam in the M61 far less important, and the emitter is more efficient. I've got two M60LL units, and I'm thinking of picking up an M61LL, even though I don't really need one. In certain ways, the M60LL/M61LL is probably the most versatile Malkoff drop-in out there.