The D&D disguise self spell is limited to +/-1 foot height difference, and remaining humanoid. That makes it reasonably close to the Change To Kind spell (e.g. on the Shifting mentalism list) or maybe Facade (Disguise Mastery). Disguise Self also affects your gear; Change To Kind is ambiguous (RMU makes it explicit that it does not). I'm not sure why True Seeing would see through it if it's an actual change and not an illusion, but I don't think we have that spell anyway. It's not going to fool a Detect or Type spell though. Also note none of these spells are going to grant AT, stat bonuses, etc, they are purely cosmetic.
I'm not sure how you balance races in RM2. Maybe there is not a system. In RMU you would assign a DP cost (probably 20 DP in this case). In RMSS/RMFRP, you'd assign a talent point cost and then use the rules in GM Law. RMFRP Character Law has a very limited Shapechanger talent (takes 1d10 minutes, only one form) for 25 points, so maybe 40-50?
I think there's something to be said for making it a racial spell list instead, so the characters start with a limited ability but can spend DP to get better at it. Thinking about the things we do as humans, there are very few that we are equal at and have no ability to improve. But that doesn't model the original as well.