You have a couple of choices. You can replace the existing key switch with a regular ignition switch that has that spring loaded start position, or you can add a separate push button switch that is used to kick the starter.
Marine stores carry nice brass ignition switches, generally made by Cole Hersee, that come either way. If you install one of the ones that has the start function built in all you do is run a wire from the "S" post on the switch to the part of the starter that excites the starter motor. (not familiar with Mopar starters, so not sure what that looks like)
If you install a separate push button switch you wire it so that it gets juice when you turn the key to "ON" and then you run a wire from it to the starter. Or, you can make that push button hot all the time , which is convenient when you want to bump the motor over without it starting.
Either way, easy to do.
Don