I had the opposite problem of the set in Michigan with an ex USAF, Morrison, 1800 RPM 60 HZ FR2 set which is rated 10 kVA at that speed, it`s an electric start with a very similar alternator/DC setup to the SOM alts, with some tweaking of the excitation system it ran fine at 1500 RPM 230 Volts 50 HZ although with a lower output. Not had much success with a Lima Mac 16 KVA 60 HZ alt run at 50 HZ these seem to have a HZ/Volt relationship that is cast in stone, another one is a Markon 4 KVA 110 Volt 60 HZ alt with 24 Volt starting windings and transformer regulated, it has a low 210 Volt output series connected at 50 HZ, a possibility with this is to bypass the transformer and fit an AVR.
The irony is that these 60 HZ alts would be fairly desirable 3000 miles over the pond.
Depending on the regulation system of the Michigan alt I can`t see why it should not be possible to run this set at 1800 RPM for 60 HZ operation.
Mark.