Take a healthy cutting, strip the bottom leaves, allow to callus a couple days, put base in potting mix, water once and not again until you have new growth. Any size cutting will root, even down to a single leaf.It is an outdoor tree. How to root cuttings? I would like to have smaller trees so the cuttings may be the way to go.
If it is healthy, yes. I think the rot might have traveled up the stem in which case no. Bare in mind that much of the tip growth is not healthy. It is probably experiencing drought at the top of the tree because the bottom cannot transport water any longer.If you cut off all the rot and wait for the plant to callus (could be a long time for that size wound), would you be able to root the whole thing in one piece?
Edit: Sorry, just saw that you'd like to have smaller trees.