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Why not just aim for a used for your "Big Block" conversion?
Would save you quite a few bucks for parts
I just stubled on this, which doesn´t look bad to me:
https://www.ebay.com/itm/WILESCO-STEAM-TRACTOR-APPEARS-NEW-AND-UNUSED-MADE-IN-W-GERMANY-VINTAGE-TOY/133698304020
Furthermore has the advantage of being old enough, to have a "real" steamvalve on there. -