You should probably leave the suckers (what you call sappers) and let them get big enough and cut away the old and diseased trunk.

What minnamouse said -- if you don't let any new suckers come up, then when the main trunk rots out or splits, as it will eventually, then your beautiful old tree is gone! If you let some new growth sprout up, it will continue after the main trunk is history.

I forgot to mention that if you like the tree look so much, after a while, you can start grooming the suckers so you establish a main trunk. I mean do this after you get rid of the old trunk and the bush proves itself able to survive the infection/infestation it has.

I have dug some suckers up and potted them to move and plant in a different part of the yard. I know the trunk will die out eventually, I just wanted to keep it around as long as possible. I don't want to cut the old trunk away, that would be killing the plant. If I keep the suckers trimmed back it will keep the main plant stronger yes? If and when the tree dies I don't mind if there not a lilac bush there, I'm hoping to create a few new ones around the yard in the mean time. I'm not that attached to the tree shape this old boy just happens to have been grown that way.

Thanks for all your input and advice!