If you want the indoor boiler to stay hot all the time you will need to add the pump. If you happen to have a pump on the indoor boiler that runs 24/7 (or could be changed to run 24/7 without screwing up the system) then just put the heat exchanger in series with it.
If the indoor boiler does not need to stay hot, you can probably use a strap on aquastat to disable it when the OWB pipes are hot.
The pump would depend on heat exchanger size. Most likely something like a Grundfos 15-58 will work since there isn't much to pump through. If you can keep the pipes large and have a large heat exchanger a Taco 007 will move more water.