I don't know where your Modus (VP?) BB falls in the quality range, but suspect its somewhere in the low to mid. If it's running smooth, quiet and without play, there is no need to replace it.
Square taper cartridges today are still available in categories that might best be called cruddy, plenty good, and quite expensive. Cruddy would include the very cheaply made units that come as original equipment on very low end bikes from dept stores and the like. Plenty good would include Shimano UN54 & UN55, Sugino, IRD and some others. Quite expensive would be the likes of Phil, White Ind, and SKF; these run up to and well over $100.
Plenty good is what I'd recommend. Shimano UN54/55 are readily available everywhere and have an excellent reputation. They're not particularly light, but the bearings and seals are excellent. They run $25-35, and are about as good as you'll find this side of $100+.
Not aware of any 121.5mm cartridge spindles. But 122 is a common size and will function identically. You need to choose the correct shell width, 68 or 73.
Last edited by desertdork; 07-06-12 at 09:22 PM.
Reason: Posted before finished