Pete,
Sorry to hear you have arrived at this crossroads, but I am sure there is a solution where you can eat your cake and have it.
Have you thought about giving the car to your sons, asking them if they would like to have it?
You never know, they might jump at the chance of having dad's turbo, and may even finish it sooner than you might expect... So make sure the deal includes you having visitation rights on ace cafe nights and occasional blasts

You might also find that helping them to complete it, without you having to complete it, or it being in your way, you might enjoy working on it with them as a distraction from your steam engineering.
You're old enough and wise enough to know this, but whatever you do in life, you need to be happy and please yourself sometimes, if steam engines will rock your boat then go for it, just don't do anything you might regret, like selling your turbo and ending up wishing you had one last blast in her before she went away with a new owner.