There is documentation for the MI instructions, but that generates an OPM program and not an ILE program on IBM i. Sadly the TIMI isn't really documented anywhere - as well as ILE.

I have written an RPG3 interpreter with all the Record Level Access operations working on a noDb/flatfile in C#, but lost the source after replacing a hard disk. I have also tried writing a RPG4+ (only free format) compiler for .NET, but it started getting tricky when starting to write the RLA operations because they relied on a database. I have the code still, but it's ugly so I never released it.

Would be a fun project, for sure. Not enough hours in the day.