If it's got one pad retaining pin it's most likely a Brembo F05 (sometimes called a P05). Get in touch with Selwyn Motorcycles Tel: 01223893331 or there are guys selling them on ebay. The important dimension is the distance between mounting point centers. Brembo haven't made the F05 for ages and Guzzi swapped to an identicaly sized Grimeca unit in about 1993. The Grimeca caliper is available for about £90. Seal kits are available on ebay and elsewhere and there is a guy in Denmark making pistons and selling them for £9 a pair also on ebay. I made my own pistons over the last month and it is quite an effort taking about and hour and a half each. I also had to make a tool for the outer seal groove as I couldn't find a 2mm parting off tool. In 1980 Brembo changed from chromed pistons to aluminium ones so if buying second hand try to find the later one.
No idea which Master cylinder was used but the 12mm bore seal kit is still available. Unless worn out and leaking many laid up brake systems just need dissembling and cleaning.
Hope this helps