Hi,
I've been using a KTM valve spring kit for their ATV 450/525 engines, part# 83036028044, for one year now in a FC550 2002 engine installed in a MX-sidecar. No problems, stock 37-30mm steel valves. To have them fit the pre-2004 heads, you'll have to file away some of the head and rocker arm cover.
Best regards,
Stephan