My bike came with the standard dual seat, but I just felt a British bike of this vintage cried out for a solo seat (The original tandem is safely stashed up in my attic for the next owner!).
When I went to fit the new solo seat, it was clear that the "stock" position wasn't going to comfortably accommodate my 6 ft 2 in height, so I fabricated an extension for the front mount. It took me 3 iterations to get it right so that the seat is very stable but the forward mount pivots on its axle to accommodate the spring compression at the rear (what is visible is just a light gauge metal cover hiding the much beefier mount underneath):
Of course, this rearward placement of the seat (which now fits me perfectly) required extending the mounting location for the springs at the rear. I fabricated an extension bar of 1/4" steel stock, and it has proven to be very solid and sturdy:
Finally, when I had the rear mudguard rechromed, I decided to weld up the now-unused mount holes in the top. Later, I decided to add an aftermarket pillion pad (partially to cover my less-than-perfect weld job). It is attached solely with very strong 2 sided mounting tape. It is strictly for "show" as I never intend to have a passenger on the back:
As you can see, there are lots of departures from pure stock on my bike, but all in the mode of what I'll characterize as "period correct"
For Gui.Dorey: I have been following your own issues with your blocked engine. My heart bleeds for you, as I know first hand just how much effort you've gone through to get to this point. Don't be discouraged, persevere.. there will be light at the end of the tunnel, and in the end it will be worth all the effort.
FWIW, I've documented some of the tasks involved in assembly of my own engine on my personal blog.. you might find something useful there:http://blog.auldridge.org/2018/01/16/the-queens-revenge/http://blog.auldridge.org/1953-ariel-square-four-motorcycle/
Phil Auldridge Austin, Texas - USA
Visit the North America Ariel forum: http://arielnorthamerica.org/phpBB3/
53 SQ 4 MK II, '72 Norton Commando, '78 BMW R100/7, '79 Honda CBX, 2014 Indian Chief Vintage