Honda-SOHC

I'm having a few problems with dirty fuel and think the tank has rusted inside with standing (there's no sign of the tank having perished).  It has had a good clean out with caustic soda and well rinsed after but it seems that there's still rust inside which is getting into the carbs and now clogging up the inline filter I've fitted.

I've already had the tank sprayed so don't really want to buy a replacement. 

What are my alternatives?

I'm struggling to source the two ball joints for the gear change linkage.  David Silver have the gaiters but not the ball joints.  Any suggestions?  Perhaps another marque has the same type?

Thanks guys.  I'll stop worrying then.  I've found that going down the gears one at a time and releasing the clutch each change, instead of block changing helps, although finding neutral isn't always easy. I may just try the gear linkage, it needs tidying up anyway as one of the gaiters is split. 

Just some general advice needed.  My 1977 400/4 becomes difficult to select 1st gear and find neutral when it's warm.  It's ok for the first half hour (ish) of riding but becomes increasingly more difficult until it's nearly impossible.  The bike has less than 19,000 miles on the clock and has been consistently on the road over recent years but has covered very little mileage in that time.

Any suggestions as to the cause or a cure?