You probably have a load of clag in the catalyst. I would take it for a good long run, to burn out all the rubbish and to wake the whole system up. You don't need to rev the nuts off it (as some will no doubt suggest!
) but make sure teh beast has got well warmed through. 20 - 30 miles is good.
If the cat' has taken offence at being left sitting in the cold/dark with just rich-burnt petrol byproducts and condensation for company, it mayhave been damaged and no-longer doing cat' things. You may need to look at a replacement catalyst.
Also maybe likely is the lambda probe. The beast doesn't like sitting unused for long either, especially if the engine is running rich (cold-starts). Again it may have got the hump/corroded. At least these are easier/cheaper to change than a cat'..
Normally if the lambada' is knacked you also get a lumpy idle etc. so you can tell if the lambada or the cat'. Thegood thing about cat' is that once the rubbish on the platinum bits has been burnt off, the beast is back to normal, so persevere a bit before you rush out and buy a new one.
Brake pipes will all be on their way to Boot Hill by now and genuine AR parts are getting like rocking horse poo. But you can get copper pipes made up to replace the offending ones.
All the pipes going in to the calipers will be the worst affected.. but check the lines that run to the rear calipers too..under the front bulkhead where they bend round under the floor is well-placed to get shot-blasted by the front n/s wheel.