We've not seen an O.S. engine with the designation, "FS-18SR". Are you sure it's an O.S. engine? Perhaps it's one that was made especially for Tamiya, and available only through them.

If the engine has the mixture properly-set, then it's possible that the piston/cylinder fit is starting to erode and you're losing compression as the engine warms. That would be a result of a possible overheat at some time, foreign object damage, dirt ingestion, corrosion, and so on. The engine may need a teardown to check it out.