The C25 is very similar if not the same as the older Mk25 Villiers.
Remove fuel tank and engine cowl; undo cylinder head bolts, remove cylinder head. To access valve springs remove cover on side of engine located below inlet and exhaust. To remove valves compress spring and remove collars, this can be a bit of a fiddle. Re-assembley is reverse of dismantling; to set valve clearances check exhaust with inlet fully open and check inlet with exhaust fully open before fitting valve springs. If valve clearance is too small end of valve stem can be ground or filed to increase clearance, make sure end is square and flat. Meetens may have owners manual instock, this will give details of spare parts and routine adjustments, but not a full engine rebuild guide.