I have a 30 year old natural gas ICG furnace with air with a one speed ac motor. Is it possible to change to a two speed, or a variable speed motor?

Yes just buy a new furnace. The cost associated with what you are thinking are not cheap so you would be better served energy savings wise to get a new furnace with the options you want.

The conversion can be done but it will be best left to a local contractor since it involves control system modifications along with safety concerns.