17670's will definitely fit. These have another advantage over 2xCR123 & 2xRCR123 - most LED modules will be only semi-regulated at the voltage range of a single LiIon rechargeable, so output will be slightly less but most importantly, generated heat will be less compared to a two-cell setup.
Please keep in mind what others pointed out previously; that there aren't many choices for reduced-output LED modules compatible with the poorer thermal properties of the G2. The best choices would be Malkoff, Nailbender, and perhaps KosPap.
[...] I'm okay with 60-80 lumens as long as I get a good 4 hours + of usable runtime with it. Since the original SFs came with 80L in plastic bodies, am I safe to assume as long as that output (and power usage as a result) is kept, it'll be ok?
If the G2 has a Nitrolon head/bezel assembly, maximum output for your light without risking overheating the LED module is in the Malkoff M61
L range, ~120 lumens or so.
If it has an
aluminum Z44 head-bezel assembly (more common with the SF G2 LED versions), the light will handle outputs of up to ~200 lumens or so. Gene Malkoff has stated that the full-power Malkoff M60/M61's should be OK in a G2 configured like that (i.e. w/ the aluminum Z44), but I'm pretty conservative so I'd consider that to be borderline. The Malkoff M61's run a touch cooler than the older M60's, so an M61 with a single 17670 LiIon would be about the maximum that I'd consider.
Again, if you don't want to push it with regards to heat, a Malkoff M61L would be a near-perfect module for the G2, ~120 lumens for more than 4 hours. (IIRC PoliceScannerMan got ~10 hours of runtime from the M61L and two CR123's when the module first came out.) Far far far better than the original SF P60LED module from Surefire - more lumens, more runtime, greater reliability, greater resale value, and less heat.
If I had a dollar for every SF G2 out there with a Malkoff 'L' or 'LL' in it, I wouldn't have to be here on CPF at 6:AM on a Sunday morning, lol. ;-)
Best of luck, :wave: