That 'man cave' sounds great. You could set up a London classic MV repair shop there, with all your experience from the 'Flying Ant' i'm sure it would be a success.
I think the only way you could fit a single seat on yours is exactly as they have done on the ebay bike. Get a suitable sized saddle from one of the vintage suppliers, measure up and get some sturdy brackets made. I must say i think the little MV's look great with the single seat, especial with a little rack behind. Not to good for offering pillion rides though!
If you struggle finding a saddle drop me an email.