Download the service manual from the link in my signature block below. Section called oil cooling system has a diagnostics tree. I have an 86; designs are similar.
The temp sensor may be out of spec. Should be ~10.5 KOhms. If higher, the fan doesn’t come on early enough; if lower, fan comes on too soon.
If similar to mine, the sensor should read 1.05 To 0.95 KOhm at ~100C and fan should come on—but no light.
at overheating temp the oil light and fan should come on. Since yours skips a step I’m thinking sensor and fan controller. Your FCU may be integrated with CDI.
Ask the members at atvhonda.com—they’re very knowledgeable.
use the oem parts finder at Rocky Mountain atv and Partzilla