Actually I found a new MM in my stash. A McR-18 does fit but it sits loose inside the head. The socket (or emitter in LED mods) would have to hold the McR-18 against the reflector. Again, don't know what your howire socket looks like but don't think this would be a good solution for your needs.
I think boring the head to accomodate a McR-20 would be best since it's secured in the head. But as mentioned, not sure how it will work with the socket & focus with the incan bulb.