Ah, OK.
First thing I would do is take off the airbox and take plenty of pics of both the airbox and the carbs themselves, including the damage if you can see any and the rear of the carbs (intake mouths) with the airbox removed, and post them here so we can have a good look at them.
The fire could have burnt the plastic throttle slides, slide guides, slide diaphragms, and even warped the caps/lids.
The slide guides, although relatively expensive @ 22 quid each are available new, but the slides themselves are not (as far as I can tell), hence the Mechanic saying look for second hand.
Here's a good source for parts in the UK -
http://www.allensperformance.co.uk/triumph.html
....although you can also get parts for the Mikunis from forum member "Moto" in the US too.
So if you can find all the parts you need to rebuild it/them, all is good but you might also want to think about cost. If it's still going to be costing you heaps you might want to consider changing the Mikunis for Keihins, if you can find a set 2nd hand, as they are less trouble than Mikunis and allegedly get better mileage too.
Several forum members have done this already so there's plenty of info available here, and I must say that if I could have found a set at the time I rebuilt my Mikunis I would have swapped them out in a heartbeat.
But all is not lost, don't despair, rebuilding is not that hard as long as you can find the parts and I'll be happy to help where I can.