I have not checked the requirements, but I seem to recall that adding another class/type to your IR privileges only involves doing an IR exam in the relevant class or type. You have already demonstrated the needed skills when you obtained your ME IR(MPA). I hope someone else can clear this one up though...
Edit: I just had a quick look. FCL.620 IR states:
Applicants who have completed a skill test for a multi-engine IR in a single-pilot multi-engine aeroplane for which a class rating is required shall also be issued with a single-engine IR for the single-engine aeroplane class or type ratings that they hold.
So if you were to do a ME IR(SPA) now, you would get the SE IR for free so to speak. In your case the ME IR is stil valid, just not for MEP or SEP classes.
I think that the bit in Appendix 8 'Cross-crediting of the IR part of a class or type rating proficiency check' may be relevant here: You are the holder of a ME IR(MPA) rating and you would be able to cross-credit the skill test for this to the IR part for a SE class rating '
Provided that within the preceding 12 months the applicants have flown at least three IFR departures and approaches exercising PBN privileges, including at least one RNP APCH approach on an SP class or type of aeroplane in SP operations'. You have not flown those three departures and approaches, but you can do that with an instructor on a relevant SEP, and then do a skills test for an IR on a SEP type to renew the rating.
The three approaches or whatever you and your instructor deem to be necessary would be considered as the training necessary before renewal. The IR skills test on a SEP would then get the SE IR part reinstated. Or, if your ME IR(MPA) is due to be renewed soon, you could perhaps use that and cross credit the SE IR part as stated in Appendix 8.
Right, this may all be wrong, but the short version is still that I don't see the need for a full 50 hour IR course.