I’m constantly switching between ‘New Parts’ and “Merge” in the Midi Record Mode menu at the bottom right of the project window. Is there any way to assign a key command to those? I don’t see them in the menus or in key commands anywhere.
Nevermind, found it. You can only toggle through the options with a key command.
There is actually a way to toggle between two record modes 
First you assign a Keystroke (W) to Transport>Record Mode.
Then create a macro:
Record Mode
Record Mode
and assign a different keystroke (Shift-W) to it:
Now, by virtue of there being three possible modes, you can toggle between two of them.
Start with, hit W to switch to Merge, and shift-W returns you to Replace.
A cool thing about it is that you can switch between any two modes depending which one you start with.