Depending on how your boiler is cracked, you may be able to effect a repair with JB Weld or similar "glue". Also, depending on how your carbon blocks are cracked, if there is more than half still attached to the electrode connections you will likely still be able to raise steam, albeit a bit slower and with less replacement production, so you will need to run the engine slower so the boiler can keep up. This is not necessarily a bad thing, as I do prefer to run my engines rather slowly, which more naturally approximates the speed of real steam engines.
In any case, there is currently a K.J. Miller up on eBay, that has been listed before and didn't sell, so perhaps the seller would be interested in taking an offer, though that option isn't listed. For point of reference, I have seen these engines sell for as little as around $40, sometimes in pretty good appearing condition, so if you keep your eyes peeled, perhaps you can score a bargain to get your replacement boiler?!?!
It is in the Beam Engine configuration, has a box in decent shape, and has all three blueprints, but does not appear to have any of the other needed parts to build the two other configurations.
https://www.ebay.com/itm/335394073784?Good luck with your resurrection of this old steam plant, and keep us informed as to your progress!