Wow! A lot of work required there, but it'll be worth it.
I was lucky. I acquired a similar loco (Martin Evans' Rob Roy) from an estate.
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It had been sitting on a shelf for about 25 years, but was virtually unused and required only freeing of the regulator, a thorough cleaning and lubrication, and a boiler test (successful!) before I was able to run it.
Keep working at yours!