Thanks for the tips guys. Total newbie here, but found my old 6P that had been in drawer for 15+ years. I'm trying to hold onto the 6P as a primary but not sink a ton of $$ into it and to keep it simple, hence the rechargeable. I haven't researched bulb conversion, but I will now!
RE: 16650. Do I need anything else, or just the battery and charger? Is upgrading the bulb (I assume 6v) recommended?
The cheapest and simplest way to go rechargeable is to get a 16650 and a charger. You should be able to reliably get an hour of runtime per charge before the light starts to dim dramatically - but be aware that even at full charge it will not be as bright as on the pair of CR123A primaries that it was designed for.
I recommend that before investing in li-ion batteries and chargers, you may want to buy a 6-pack of CR123A batteries and just replace them as needed. This would give you 3 hours of actual use before needing more, and in that time you may decide that you would rather upgrade or replace the light rather than put more money into it as a primary user.
How much money are you thinking?
The 6 CR123A batteries would be around $12 online.
A single 16650 and charger looks to start around $25. That would be all you have to spend to get an hour at a time of dimmer incandescent light.
A Lumens Factory incandescent bulb that is designed to run on li-ion is in the $15 range if you want to stay incandescent. It would be bright but still limited to around an hour runtime.
A Malkoff M61WLL is an LED dropin that would mimic the incandescent P60 in appearance, but with literally ten times the battery life. It costs around $50.
With a 6P you have many many options.