Welcome to the forum J. There are a few after market base plates available. Lee Valley sells a couple I think. You`ll also need a centering tool for lining the new plate up with the router. Bosch`s cone style one is the most versatile and is only a bit more money.
An even better idea would be to keep the old Stanley for the jobs it is good at and get a new plunge router to use with the template guides. The new router will already have the appropriate opening in the plate or an adapter to hold the template guides and some of my routers came with the centering tool also.
Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.