Clean the commutator.
Make sure the brushes are adequate. (replace if necessary)
Clean (lacquer thinner works well) the bearings and lube with plain
oil (NOT any kind of Penetrating oil!!!)
Make sure when reassembled that it spins freely. If it doesn't, clean
and adjust until it does.

Most heater blower motors have not had any attention in years and are low
on power because the brushes don't make good contact or the bearings are
gummed up or both. There is nothing about the power-producing part of an
electric motor that makes it get weaker with age (unlike piston engines!)
--
George Downs, Bartlesville, Oklahoma, Central US