From the pictures you've posted your collectorbox doesn't look at all bad.. Take off the end-can and the front downpipes and have a proper look, it should be repariable.
If the guys never changed a collectorbox before I'd work on at least 6 hours of labour.
I have an MTC Stainless Oval exhaust and link pipe somewhere I may part with fairly cheap if your going after-market pipe.
Did you have it dyno'd / rejetted when you restricted the bike? Running that rich isn't good for the engine, your going to be using far too much fuel, and will foul the plugs and valves up which can make the bike hard to start.
If you have slack in the choke cable it might not be the right one? What do you mean by slack though? You can pull the choke lever all the way down and the choke isn't on? Theres always about 3-4mm of plunger than you can pull out more than the choke cable will pull.
Most important things to ensure its runnign right are.
1. Valve clearances checked and adjusted where needed, the NC24 uses adjustable tappets so this is an easy job. Always use two feeler blades to get a more accurate reading.
2. Carbs clean, specs checked and balanced. There'll be no point setting them to factory settings if you've had the bike restricted, a dyno operator will be able to measure fuel / air and advice on correct size jets. As a rough guess i'd go down from the Standard 115's and try some 110's off an 89 NC30. The carbs are failry easy to mess with once you've gotten used to them. Gettin em on and off and sealing right can be a bit of a ballache with older carb rubbers.
3. Good Plugs, Fresh Fuel / Clean Air Filter. Obviously, without air, fuel or spark the above are useless. Correct Plugs are NGK C8EH9's or ND U24FE9, although how cold its been lately NGK C7EH9 or ND U22FE9 might get the bike a little warmer
These bikes fire 180'degrees like their 750/800 counterparts and sound fairly even when ticking over and should be smooth and quiet.
Take note of druid's advice though.. I wouldn't invest in the bike too much as theres plenty about and they dont' fetch a lot, it could easily turn into a bottemless money pit.
They are great fun to ride though and the 16 / 18' wheels make it 'fast' around the bends.