I have a Yellow Flame hot water boiler that was in the house when I bought it 7 years ago. I was told it was over 35 years old then. The old gal is hard to kill.
It was in pretty bad shape as it had been neglected for a LONG time but it still churgged along. I rewired it, change the controls and timer, drive coupling and the gear box. Other than that I clean it three times a year, keep it oiled, and it burns all year round. It will do some serious long term heating. The boiler itself is sheet steel and can be welded should it spring a leak.
Marks Supply in Shenandoah Pa. bought up all the parts and the rights from Yellow Flame when they went out of business in I believe the 1980's. You can get grates, stoker motor/fan drive couplings and rebuilt gear boxes from Marks Supply. The drive motor is a standard oil burner motor. It's about as close to bullet proof as a stoker fed boiler can get.
And I already talked to Keystoker about in the future when Marks doesn't carry parts anymore. They said that you can buy a complete stoker unit from them and all you would need to do is make a mounting adapter plate for the Keystoker unit that will bolt up to the boiler.