Yes this Eberspacher would be suitable as a replacement for heating a Narrowboat with 2 radiators, does your existing heater also provide domestic hot water via a calorifier (hot water cylinder)? Does your existing heater have a 5 ltr header tank? You would require to fit the supplied new wiring harness as the Mikuni harness would not be compatible. Dependent on exiting Mikuni heater model the exhaust is possibly the correct size (24mm id) though you should NOT fit the silencer included in this kit because only suitable for automotive external mount.
Please email sales@butlertechnik.com if you have further queries on the swap, we could then double check all components, make the swap as simple as possible.
Thank you