Information based on my Quota 1100ES motor and would apply to other fuel injected models. Simply measure the I/D and O/D of the seal and search here http://mrinjectoruk.co.uk/default.aspx
A set of eight injector seals for mine was £12 delivered. Fast service and very helpful text response to questions.
The whole injector costs £300 from Guzzi. The Weber IW031 is also used on Lancia and Lamborghini cars and can be picked up for about £120.
Servicing is about £40 per unit but doesn’t address potential problems with the AMP connector.
Have fun
Steve
Also check with car spares as these are also used on Fiat , Lancia and Maserati