Just so I'm sure, there was no spring under the strainer cap? That's a new one.
The spring for a Honda XR200 should be a perfect match, as long as it's '81-'83 or '86 on. The '84-'85 models were OHC, and possibly used different strainer springs. You can probably find a replacement at
www.rockymountainatvmc.com or
www.bikebandit.com