A new tank will plainly sort out any problems (at a price) but I for one would much rather salvage & preserve an original tank than buy a new one. Certainly worth a try with commercial products but if that doesn't work out I'd take it to a specialist and get them to take a look - most radiator repair shops also repair fuel tanks. Lincolnshire Radiators have a good reputation.
p.s. If you succeed in getting it tolerably clean, a simple in-line fuel filter will take care of minor remnants. You can by them for a couple of quid on eBay.
p.s. If you succeed in getting it tolerably clean, a simple in-line fuel filter will take care of minor remnants. You can by them for a couple of quid on eBay.