I have several 1960's-70's Aurora Thunderjet 500 HO slot cars. While the motors work, they run much slower than when they were new (and I was young!). What can I do to recover the speed that they should have? Is it the magnets getting weaker? I've cleaned and lubricated all the motors, but they just are slow. If it's the magnets, is there a way to "charge them up"? Your advice is appreciated.