Aha. I believe that arm has an effective mass of 12.7g, which makes it compatible with a wide array of current cartridges. It's still a bit too light for a Shelter though. Twl and others could recommend specific cartridges that would suit. I'm thinking a mid-range model from Dynavector, Benz, ZYX...
I have some general thoughts FWIW, which I'll attempt to offer with appropriate delicacy. I wouldn't put a very high end cartridge (Shelter 901, Allaerts) on this TT and arm. I'm not saying you wouldn't hear improvements. You certainly would. But you wouldn't get anywhere near what you paid for from the cartridge. In addition, a cartridge as revealing as a Shelter 901 (I have one) or similar will clearly display the limitations of ANY table or arm. I hope you'll understand, if only by comparing their prices, that a Thorens 125Mk2 (even up-weighted) and an SME 3009 are not in the same category as today's top cartridges. The bearings and wiring of a 3009 are not the best, and a TD125Mk2 needs more than some extra mass to become a top level table. The bearing, platter and motor would all need significant upgrades too. Mounting a super revealing cartridge would highlight these shortcomings and might quickly become unsatisfying to listen to. I've done an analog upgrade this way myself. Please trust me when I say it doesn't really work, at least not for very long.
If you're committed to trying this Thorens with high end components I'd upgrade the arm before buying a top cartridge. I don't know which arms would fit, ask User510, but perhaps a higher SME would. An SME IV should be easy to find used. That would suit many top cartridges, and would transfer to quite a few top tables should you decide that's necessary. Again, I don't recommend this approach. But I think it's what I'd do if "compelled".
Just my $.02.