I think the seat/rear mudguard might be aftermarket but the tank looks right. The radial head comes with the bike-you can see a photo of it if you scan through the photos. Correct reproduction 'glass can be bought from this forum's Flowerpot Racing. I just bought a full set including tank for my genuine Rickman Petite Bultaco frame. You won't get better 'glass than this stuff, it's ethanol/methanol proof too. My list has the engine as a 200cc Campera, made from 1963-1965 with engine numbers ranging from 900.001 to 921.899 which would positively identify the bikes M-900826 numbered engine as a 4sp legal pre 65 set of cases at least. I seem to remember Anthony Suttor converting a Campera to 250cc for pre 65 back in the 90's so it must be possible.
The pukka Petite Metisse tail piece comes to a point and the subframe has a fitting right at the edge to bolt the rear most tip of the rear loop to the subframe. If you look at the pic on ebay there is a bolt thorough the rear most tip of the guard that will probably line up with this fitting. Mark, your rear loop has this fitting and the tail piece you are getting is the correct pointy type. The pics on ebay look lieka Mk3 rear guard has been used. Personally I prefer the more rounded look of this guard. It is harder to fit this guard on the Petite rear loop, hence it is looking abit ouit of alignment.
It is a race bike and has probably been used in it's 40+ years and I dare say at some pint it has been looped making matters worse.
Seat shape is not correct as it has been extended at the front to protect the riders crown jewels as the gap between the tank and the seat can catch them if you are not concerntrating. A good idea and will help to may well help make it even more ridable.
Frame should be plated if it is a New Milton Rickman (531 tubing etc) not painted (normally which means a repair).
The Spanish copy was made from a poor quality tube and was painted (like your pickture Mark) not plated. They are renowned for breakages so always check them carefully if you are builg one and planning on racing it.
Age? Mark look in your book, all the builds are down there, so you know who it was first sold to.
Only thing that really isnt right is the exhaust. Should be an up and over. But who cares? Not me, it's a lovely looking race bike and at the sale price I would have through that it is cheap.
Head is the same as my Pukka Petite Metisse (made in England). Not all of them had the radial head like that (which was also fitted to various other models; trials and road) they all look similar (to me anyway!) and are different.