Garageboy,
As Greenled said, the McR18 is a direct drop-in which will not need any work done to the reflector to fit in a lux mod for minimag. The only thing to improve, is to add in an o-ring to keep it from 'wandering around too much'. (for the o-ring size, I don't have one handy, but check them out in Wayne's sandwich shoppe)
The McR19, IIRC, is a longer reflector and will not fit into Minimag without cutting/sanding it shorter and trimming, with that kind of workload, I would then recommend modding the McR20 to fit it in(the how to is detailed in my guide)
As for the SMJ, you too could use the McR18 to acheive better performance than the stock minimag reflector, but the fit will be quite loose and an o-ring will be a must to keep it somewhat centered.
And if you decide to use the stock reflector, sanding/drilling will be needed to open up the reflector--do kindly check out the section in my Dummy Reference Guide to Minimag mod, there is a part which details how to drill the stock reflector to fit the lux, the opening for the SMJ will not be as large, but the idea is the same.
Good luck with your mod!