Oh, I see, snapshots for Forge 1.13.2 has been released \o/.
I've started updating the RM to 1.13 (with Forge built from source) a few months ago but was eventually blocked by the Mixin library not yet working on 1.13 (and we really need it). I am unaware of it's current state except that no new commits have been pushed to the main repo.
I do plan to resume the update process once the Mixin lib is available. So, if you wish to help, getting Mixin on 1.13.2 would be a great.
For producing usable RM jars, we'll also need MixinGradle to be compatible with FG3. It should not be necessary for working on the update though, so it's secondary until I'm done with the main chunk of update work.