GT Motorcycles are based in California and offer all sorts of Guzzi goodies:  https://gtmotocycles.com/

At last, a British made stainless steel bevel/gear box breather unit is now available for Moto Guzzis. 

Manufactured by Chris Shaw Engineering, they're available online @ £18.95:

https://www.shawstainless.co.uk/product.php/7049241/

In alphabetical order, a list of companies that offer parts and/or service facilities to UK based Moto Guzzi owners and enthusiasts.

Chris Shaw Engineering - Online Special Stainless Parts.

Tel: 01790 755 515 

Website: https://www.shawstainless.co.uk

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Fowlers Parts – Online Parts.

2-12 Bath Road

Bristol

BS4 3DR

Tel: 0117 977 0466

Website: https://www.fowlersparts.co.uk/browser/manufacturer/moto-guzzi

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Gutsibits Moto Guzzi Spares – Online Parts.

Unit 30 Uppermills

Canalside

Slaithwaite

Huddersfield

HD7 5HA

Tel: 01484 841 395

Website: http://www.gutsibits.co.uk/pr/Home/index.php

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Moto Corsa – Service & Online Parts.

Unit 6 Souths Farm Yard

Noade Street

Ashmore

Salisbury

SP5 5AA

Tel: 01747 811196 (Service: Option 2 or Parts: Option 3)

Website: https://www.parts-motoguzzi.com/

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Stein-Dinse GmbH - Online Parts.

Waller See 11

Schwülper 38179

Germany

Tel: +49 531 123 3000

Website: https://www.stein-dinse.biz/Moto-Guzzi:::1.html?language=en

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Teo lamers – Service & Online Parts.

Lagelandseweg 66

6545 CG Nijmegen 

Netherlands  

Tel: +31 24 371 1111

Website: http://www.tlm.nl/en/