Is there a Bachmann coupler that will replace the MT coupler in my GP38 by Life-Like?
For what purpose? The MT coupler is the defacto standard for N scale currently.
What type of coupler do you want to replace it with? Hopefully not Rapidos. Rapido couplers are no longer supported by any of the manufactures so any new product will be very difficult if not impossible to switch to them.
If it is a newer Life-Like / Proto N loco and you want to go back to Rapido couplers, you really don't have an option. The older "Train Set" GP38's with truck mounted couplers came with rapido's so you would just need to reinstall them. This is only possible if somebody hasn't modified the trucks to install the Micro-trains couplers.