If the 520 coal boiler is being used to provide hot water for another remote (oil, gas), functioning boiler, simply install an L6006A aquastat/or L6006C strap-on, to the remote boiler. Then have that L6006 aquastat control the circulator at the coal boiler.
Set the remote boiler's (triple) aquastat low limit at about 150 and the L6006 at about 160. It will take experimenting to get the right settings. The coal boiler will maintain its own temperature via its own (triple) aquastat and will circulate to the remote boiler as long as there is demand from the remote boiler.
In the event you have an outfire at the stoker, the remote boiler will kick in and take over. An outfire control should be installed at the coal boiler.