Will this fit the OHC Cover Head on my Honda HRB425 CQXE lawn mower, or do I need a different part number ?

There is no gasket fitted on this it is a sealant that is used. The Honda sealant is part number TB1207B, Liquid Gasket 100ml for Honda GHX50 GX100 Engines https://www.lsengineers.co.uk/catalog/product/view/id/9947