Drain the oil and replace. The kit probably gives you a measurement from the top of the fork to the oil. Use that, not the volume (cc's). You changed the amount of components inside the fork so now the fork oil level is probably wrong unless Matris specifies the same as OEM Guzzi. Use what Matris specifies, not the Guzzi manual anymore. Save the instructions for future reference. Besides, you can put in some good synthetic fork oil and improve your fork action while at it. Pulling the fork tubes really isn't hard other than supporting the front of the bike.
Edit: 20wt?! Not unless specified by Matris. That sounds heavy to me but maybe that is what they call for.
Edit: 20wt?! Not unless specified by Matris. That sounds heavy to me but maybe that is what they call for.