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You can try Skinner in Erie, Pa. They may still be building reciprocating engines for export. I doubt that they have much of a selection of smaller engines though.
And, as I already suggested, many "experimental", or one off, high speed engines have been built using the Tiny Power M castings as a start.
OR you can always build a roller/ball bearinged, enclosed crank, forced oil, poppet valved, uni-flow from scratch. If you design it properly, you'll have something as good as anything currently in production.