Last time I needed a mono cartridge, I wired a Shure M91 with the two channels in SERIES (ie, + of one channel to - of the other, and used the two remaining leads as outputs), and it worked just fine. Gives a little extra gain, which is frequently helpful with vintage tube mono preamps/amps/integrateds.
I would think that this procedure should work OK with most cartridges...
Regards,
Gordon.