No, the RS250 cranks arn't the same. They have either the wrong center main bearings (depending on which MC18 cases you're trying to fit them in) or the wrong dia flywheels. They also have smller little ends for use with HRC pistons which are shorter than stock pistons so require barrel modification to make them work.
Add to that '90- have the wrong crank taper for the flywheel.
Apart from that they are a easy swap.
MC18 cranks are only ever going to get harder to find so if you find one treat it well.
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If I have to take the carbs off once more...