The plans "A Stirling-Powered Tractor" were published in "The shop wisdom of Rudy Kouhoupt" volume 3. Rudy mentions it is "About 1/12 Scale".
Some dimensions To give you an idea...
Frame rails 13-1/8" lg.
Rear wheel ( rims) 4-11/16 Dia. X 1-1/8 wd.
Front wheel (rims) 3-1/16 Dia. X 9/16 wd.
It can be operated to disengage the drive wheels with a lever.
Might be a bit small for adapting a small IC engine to, but could be scaled up. Often wondered about the "Webster"....
I have just been asking the same question on the Model Engineer and was directed to Smoke Stack forum there is a bit of info but only a bit
Les Proper has one on his youtube channel
My idea was to use a flame gulper as per Les but in a larger scale to compliment my Stirling tractor
Guess i will keep looking but there is not a lot of info knocking about or I may just be unlucky