I fitted a ZX9R front end on my CSR1000 (link below). It is said to be the longest upsy dupsys available. I used a complete front end,wheel,brakes,triples etc. A lot easier than trying to mix and match. I did make my own mudguard though as the ZXR was too modern looking for what I wanted. The only real engineering involved was a stem swap which I farmed out as Im not a machinist. The guy I sent it too actually turned a new stem on the lathe and pressed it in. Job done.
On the plus side,modern suspension,modern brakes,better tyre choice and it looks peachy. On the down side, I have a slightly smaller turning radius (before hitting the tank) and the ground clearance is a concern. Im still not on the road yet so I dont know how this is going to effect real world use.
There are ways around this. The gull wing triples are one way,probably the best but most expensive.If I need to jack the front up,Ill probably go for fork extensions ( I found someone local who makes them). This only helps on the ground clearance issue not the turn radius.
There is a list somewhere (but of course I cant find it now) of various upsy dupsy fork lengths. It must be on here somewhere. There is also this;
www.newton-equipment.com/aero-600-filler-cap-model-A68sl.html
Might be helpful to you.
Another thing to think about. For myself,I couldnt do a front end conversion without also doing the back. It just didnt look right especially a modern 3 spoke wheel on the front and the old style z wheel on the back. So whatever youre going to budget for the front youll probably want to double to include the back as well.
Good luck with it.