Hi Ethan, welcome to the club. We would love to see a pic of a Zim 1200, even if unrestored.

For power and relibility, yes indeed swap the A14 from a 1400 bakkie. It'll fit right in there. You will be amazed at the power difference, and it has factory reliability.

The 5-speed will only make a difference at highway speeds -- it will increase fuel economy at 100 km/h. It won't increase your power, in fact it will reduce your power since the gears are higher.