The reduced power sounds like the usual Lycoming worn cam lobes.
Especially as you say it sat around for 3 years. All the oil drains off the camshaft in a few days and then it rusts. This is due to the wonderful idea of placing the camshaft at the top of the engine instead of underneath in the sump submerged in oil. Is a very stupid design. Can be checked by measuring valve lift. On putting a bigger engine in her the type cert may cover various engine types, so could be just a case of fitting and follow any required mods.
So I would start by reading the certification paper work.