Look in the Swap Meet for a 235 (or a 250 - a more difficult "drop in"). Or, post a Want Ad in the Swap Meet. The 235 is the easiest engine upgrade (a 261 is just as easy but they are harder to find and are more expensive).
Pertronix sells an electronic ignition module for a 6v system. The best reason for switching over to 12v is if you want to use modern 12v accessories. 6v works fine if you will be sticking with original options/accessories.
Spend some time in the Tech Tips to familiarize yourself with what is possible and how to do things.
There are "shade tree" fixes for getting a little life out of a knocking 216, depending on the reason for the knock. Most likely, a knock is indicative of future problems, no matter what quick fix might work.
