You need to match the current bottom bracket's spindle length so, if you can find a UN 55 that's the same spindle length as what you have, you're good. BTW, those plastic non-drive side cups are no problem unless you are too ham-handed with them. If you torque them too much they can break but they are not that fragile. That cup is just a spacer and stabilizer and the stresses on it are minimal.

To install these bottom brackets properly, install the cartridge end (usually the drive side) first and torque it to spec. Then thread in the plastic cup and tighten it firmly but no more. When you remove the bottom bracket, remove the cartridge side first. Done that way, the plastic cup will give no problems.