Those Magnepans are a fairly unique design. Because of that, I am hesitant to recommend anything but Magnepan to ensure you achieve the same level of accuracy and timbral signature as the 3.7's.
Mixing and matching speakers can create a very inconsistent sound field. Start with finding a suitable center channel - you will want to trial it in your home, level match the Front L and R speakers with this new center, and keep searching until you find one that blend seamlessly in with your 3.7's.
Once you find a center that plays well with your speakers, then purchase the matching surrounds for that center - good luck!