N guage engine

[macmedia]

I have an N guage engine that has small O ring on the drive wheels to grip the track.  The engine is old and the O rings seem to have dried out or are worn or dirty.  Is there a solvent that I can use to clean the rubber O rings or must they br replaced. 
The wheels on the engine turn but they just spin without the engine moving.

[ACY]

The rings you speak of are traction tires check online or call the service department to get them replaced. Replacement is not optional in this case.

[brokemoto]

Is it a steam or diesel locomotive?

If B-mann can not help you, Stewart/Virnex sells replacement traction tyres.

Replacement of the traction tyres on diseasels is not difficult.  Still, if you should need it, Stewart/Virnex does sell a tool for the diesel traction tyres.  It will work on very low drivered steam locomotives, as well, such as
the Athearn/MDC 2-8-0.

If it is steam, you might have a more difficult job ahead of you, but still not impossible.  Depending on the construction of the locomotive,  you might have to take apart rods and valve gear.  If it is a high drivered locomotive, such as the Con-Cor 4-6-4 or B-mann 4-8-4, you might have to do a series of stretches on the traction tyre before attempting to fit it onto the driver.

These things do break and dry-rot all the time.  It is not a flaw, necessarily, it is simply what happens to rubber.

Failing to replace them will leave wheels that do not contact the rails properly thus compromising runnability as well as pulling power.  The missing traction tyre can even cause derailments.  As you state, the thing will not even get out of its own way without the traction tyres.