Sounds like the classic Rotax issue most owners encounter - faulty head gasket. Head gasket and regulator replacement is an almost standard repair for Rotax engines of that era. If it hasn't been replaced in the past, then that's probably the problem. Look for oil in the radiator (but not just the overflow bottle) or water in the oil as an easy indicator, but even if there's none it is still likely the head gasket. Hope I'm wrong. Good Luck.
Brett
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