I'm probably being a bit dim but i think most of us would prefer 4 engines to 2 for a LRMPA and I think we'd accept that there should be good endurance & good low level performance. We'd probably also agree that its the "kit" & the operators that are the most important element of the MPA "system" so, given that the mission kit for the MRA4 does not seem to have been doubted could this kit not be fitted into Hercules airframes? New build, I suppose & perhaps the kit could be fitted in in a modular manner within internal pre-fab type cabins/ containers?
Admittedly I'm not sure how a weapons bay is fitted in but there is lots of experience on then type, lots of support available, its still in production etc.