I think you're trying to make a silk purse from a sow's ear. I can't see sinking that kind of money into a garden variety F3 with a single Pullmor motor. For $200-250, you can routinely find nice dual-motored TMCC F3's. The single Pulmore motored stuff that Lionel turned out usually were pretty anemic pullers on a good day.
I actually added a motor to the Lionel Phantom and then used the ERR AC Commander to enhance it's electronics. I have to say, that turned it into a totally different locomotive, it easily tripled it's pulling power and gave it much better low speed performance! However, this isn't a model you can just find a dozen replacements for, so for me it justified the time and expense.
