Computer fans do not run backwards if the polarity is reversed, they just dont run.
IF you can splice wires then that saves trying to find a fan with the exact connector which would make it plug and play.
Typically fans are 60 x 10 or 60 x 15mm thick. Thicker fans generally move more air for the same rpm and noise levels.
You can likely get for a while, by simply lubricating the bushing/sleeve bearing. Peel off the sticker on the hub and the bearing and rotor should be visible. Some might have a rubber plug to leverage out. A drop of motor oil should quiet it for an undertermined period of time.
While you are in there write down the specs on the sticker and the model number and plug that into google. You are looking to find how much CFM that fan was designed to remove and how much amperage it draws.
The replacement fan should have at least this much of a CFM rating, and you also want one with dual ball bearings, though a 60mmx10mm fan might have only a single ball bearing. The replacement fan should not draw much more amperage than the original, to be safe, but I suspect (guess) that the circuitry feeding the fans can easily provide more than the fans they do employ originally.
Often the fan will say on the sticker, something like0.12 amps and most of the time this 0.12 amps is not the running current but the startup current and actual running current is 2/3 of what the sticker says, if one actually measures it.
I have a schumacher charger that i removed the original fan and addded a larger higher rpm one on the outside of the case and it pulls air through the casing, blowing outward. I also drilled momre holes on the opposite side to allow for the greater air movement through the guts.
Cool electronics are happy electronics so you can increase the longevity of your no longer available charger, by boosting its ventilation with a better fan, and if you have some DIY skills a bigger fan moves more air for less noise and perhaps the same amp draw.
I shop for fans here, as they list 60x 10mm and 60x 15mm and 60 x 20 Mm fans making it easy to find one with the right specs and features and price. I usually plug in the desired part number into a search and see if my regular online purveyors of such items are also selling it.
http://www.frozencpu.com/cat/l1/g36/Fans.html?id=UQAYuR4INote that fans with a fluid dynamic bearing are also desirable. they will be slightly quieter than a ball bearing fan and might also last longer. It is the fans with a sleeve bearing, like which is now in the pricess of failing in your charger, that have short lifespans.
But the drop of oil on the hub might be all that it needs. However by the time they become noisy, the sleeve bearing and rotor are likely worn, and the drop of oil is just prolonging the inevitable.