Assuming they use the same powerhead then for starters I'd make sure the fuel injector/ fuel regulator (if any) are the same, otherwise it's not just an ECU swap.
If they are, or if the fuel rail is the same and the correct injector can be swapped in, then there will need to be something in the air intake side of the engine that restricts the airflow to match the shorter injector pulse (or reduced fuel delivery from a smaller injector) to keep the air/fuel mix stoichiometric. Beyond a restrictor plate, look for any cast part numbers to be the same (for any intake plumbing and cylinder heads) and that the cam and valves are the same on both engines.
There may likely also be a timing difference even if all they use for air control is a restrictor plate - you need to find how the ECU determines the cam position to direct the engine to fire (cam position sensor or some other device) and make sure those components are common between the engines.
This is all just basic fuel injected motor knowledge, not specific to the outboard in question.
Lastly, don't expect the performance to be the same as the different motor classes tend to have different lower unit gear ratios. I'd be warry about running 50% more power through a gear box if it's not designed to handle the increased torque. Even if it holds up, you will end up needing a different propeller to keep your engine at the proper RPMs at WOT with such a change in power.
