If the 15-58 is big enough then you can use most of the ECM pump options. I would use the existing 15-58 on the secondary loop since it most likely wouldn't pay off using an ecm pump there with it only running on call for heat.
If you want a delta t pump, I would look at the Taco Honeybee. Otherwise if you just want to go with constant flow, There are several that should work. The Grundfos Alpha is a little smaller than the 15-58 but from what you have said, sounds like it should work. The Armstrong 20-20CI is the same size as the 15-58. I am running one of each on my boiler and so far like the Armstrong better. There are others out there too but not really any cheaper.