A few people do that already. Some will commission others to rehair their ponies, perform eye/symbol repaints and other specialist tasks.
Some people will buy lots of baity ponies, restore them and sell them for a profit.
Many collectors are able to do simpler tasks by themselves, such as tail rust removal and sunfading.
If you have plenty of experience and examples of your work, I see no reason why you couldn't advertise your restoring services.