Well, I don't know how the newer cap assembly mounts to the tank. But If I remember the '90 correctly, that assembly is bolted onto the tank by 3 bolts. You might try asking your Suzuki dealer to look in the parts fiche (it should be on computer) and see if the part numbers are the same. You might also email or call Chip at Ron Ayers.
The cap on my 90 1100 has 8 bolts. The cap on my 02 Busa has 7 bolts. But the spacing looks the same for seven of them. The cap on the 02 is hinged down lower. But the 02 sure does look like it would fit on the 90.
"Placky Bomber" Heard them called a few things but not that, you are talking about the C90 arent you Dave? Its Ideal for local trips in the British winters when they salt the roads the gixxer would fall apart after a week. As you know "Bombers" last for ever you cant destroy them even if you try, we call bikes like that "Chicken Chasers"
As for the LC mine was definetly a cantankerous old git but fun when it held together thats why its in bits now waiting to be restored..Oh well one day