I'd be surprised if a "cheap sugino crankset" was only compatible with itself. I think the Sugino 75 (nice higher-end crank designed for single-chainring) has ISO tapers (Campy-style) but I think all other Sugino cranks use the standard JIS (shimano-and-others-style) taper and should have tons of compatibility options.
edit: The Sugino RD crank has a standard JIS taper and you should be able to use pretty much any square-taper left crankarm with it. You'll want one that was designed for a similar bottom bracket spindle width (so the left pedal ain't lots further away from the frame than the right) but this shouldn't be hard to find.
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"c" is not a unit that measures tire width