They put out the same stuff all combustion engines do; carbon monoxide, carbon dioxide, water, unburnt oil and so on.
But you will spew out more pollutants from your car than you will ever from your model airplane engine. Even your lawn mower will produce more pollution.

Its a matter of scale. We modelers are on the almost unmeasurable level. If you are concerned go electric -- oops I forgot about all those coal fired power plants needed to provide the energy to charge up the batteries. And the batteries, where to dispose of them? Pollution everywhere!

So I guess you are dammed if you do and dammed if you don't. I guess you will have to go in for slope gliding.

Cheers,

Chip