Without a schematic, hard to say. Talking about a single MOSFET, you'll need a pull-up, and a lot depends on the value of this pull-up. Use a small one, combined with a properly chosen series termination, and it's likely to be fairly good, but then you may have a power consumption issue. Too high, and this is worse than not using anything, as your line is high impedance in the '1' state! Your existing 3V3 signal is, at least, a proper push-pull signal with around 20-30 ohms of transmitter impedance. Terminate it with a series 47 ohm resistor at the source, and off you go. If this somehow isn't robust enough, then consider a proper RS422 transceiver IC - they are widely available and will get you proper margin using differential pair.