I haven't played MERP in years, so this is off the top of my head.
You should start with ERA RMC, then edit the Groups, Categories and Skills XML files to match the reduced skill set.
Then, you need to discard the races and professions provided with ERA, and create similar versions with the costs and stat bonuses in the book.
Most spells should be the same, although I think you only get to level 10 with them.
If you want more detail, you could also edit your own maneuver tables, using as example the file provided: ERA/RMC/Configuration/ManeuverTables/Influence.maneuverTable.XML
Combat would be a little trickier. I seem to recall there were fewer armor types, so you need to replace the files provided in ERA/RMC/Configuration/ArmorTypes with only those that are needed. Maybe the penalties need to be edited as well.
Finally, for attacks and criticals, it might also require discarding the files provided in ERA/RMC/Configuration/AttackTables and ERA/RMC/Configuration/CriticalTables but using the examples as a guide on the expected format.
This part will most likely the more time consuming, unless you really know your way around techniques like OCR and Regular Expressions.
If you wanna give it a go, please let us know and post here in case you get stuck.
Thanks for your interest.