I'm going to try to make some helpful comments here but note that I have never done this conversion. However, I do own the original works prototype Knight and can see some of the things they had to do. First, it depends upon whether you wish to continue to use the original UFM. oil tank etc, or try to make a Victor. If the latter then you can use the original wheels, forks, engine/gearbox and so on but you will have to buy, and modify, a 'D' UFM, rear end, oil tank, and rear frame triangle (more on that later) and some method of supporting where the rear cylinder would be on a twin. If you wish just to fit the rear enclosure on to the original UFM then you will find these problems, There might be more. You will need the long stud and special bolts etc which act as the pivot and locators for the rear frame and support for the lower front end of the rear enclosure. You will then find that the top pivot for the upper front fixing of the rear end is too high and you will have to make something up to extend the holes at the rear of the 'C' UFM upwards to allow for this height difference. This does not immediately solve the problem of the fixing point for the hinge at the front of the 'D' saddle but some ingenuity might get round that. You are now left with at least one other problem.. The rear cowl is held in the 'up' position by a prop such as holds the bonnet/hood up on a car. On the 'D' rear frame triangle there is a hole for this purpose. On the prototype, which was built using a 'C' RFM, a piece of tube had been welded or brazed onto the top of one of the RFM upper tubes to take the lower end of this prop. You now have to consider the bracket which supports the silencer. On the prototype this was made of part of an original aluminium rear footrest plate and part of a rear brake torque arm. When I bought this bike, well over 50 years ago, I had no idea it was the prototype and so I replace the fabrication with the proper 'D' component. To ease the worries of those who think we should preserve such artefacts I have saved many of the parts which I took off to replace with more robust items. Do you wish to use the 'D' centre stand? If you do then you have to obtain all the parts to go with that including the pivot plate. I'm pretty sure that I have seen 'C' mudguards used inside 'D' rear cowls. However 'D's had a short steel mudguard at the front with an inclined flat piece of steel welded to it. The prototype had this fabricated from part of an aluminium mudguard to which had been welded a flat piece of aluminium plate. Once I started using the bike this fatigued rather quickly and was replaced by the standard 'D' item. It has long seemed to me that this would be much better if it was replace by a fibreglass component which could be made wider to allow wider tyres to be used and could be moulded in such a way that it was also the chain guard.
Over to you.