My Trident has a steel petrol tank, I have just checked by sticking a magnet to it.
The tank on my bike is (was) painted internally, this paint flakes off. When I re-fuel the thing (a frequent occurrence) flakes of paint can be seen swirling around in the tank. Coincidentally, whenever I have checked the fuel filters in the carburettor junctions they have been clogged with red gunge. That may be the source of the muck in your carbs.
'Rotten' fuel tank sounds a bit vague, could you explain further?
Secondhand parts for these bikes often show up on eBay. Whole bikes can be had for what seems like very low prices.
here is a link to a fuel tank for sale;
http://cgi.ebay.co.uk/LOOK-GENUINE-TRIUMPH-TRIDENT-750-T309-FUEL-PETROL-TANK_W0QQitemZ320232838592QQihZ011QQcategoryZ10486 QQssPageNameZWDVWQQrdZ1QQcmdZViewItem