So I have a fuel tank from a 74 DT-250-A it   a locking
any one have any sugesstions ? I am going to SH-can the old cap if or when i get it off and replace it with a non locking cap

Have you tried the key for your ignition switch in it? I've just had a look at mine, as well as pictures online of a couple of others, and the lock isn't numbered. It must therefore be very basic, with any numbered key from a 1974 DT opening it (looks like that lock style was one year only). Or, if you are close to a locksmith, you could take the tank there and see if they have a key that fits it.

You must not have an ignition key? I have analyzed those locks and found that the fuel tank lock opens on the "base" part of the key. Tested and found, an UNCUT key will open the tank and is not locked as indicated in prev post.

Maybe just my stuff, but a screwdriver works too.

So I tried the screwdriver and it worked !

I love a fix that is cheap as chips! 

The way this thing was buggered up I never gave the most simple thing a shot 
thanks for that…any leads on whether the lock cylinder can be replaced ? 

On some gas caps there is a numbered code ie. 279. There are online services that can make a key that fits for about $10 USD. I have had success getting working keys for my bikes. I don't know if that is only for vintage bikes only. Good luck. I see that you have applied "the screwdriver method" with success. Sorry for the late response, as I have experienced difficulties with my computer.

"The way this thing was buggered up I never gave the most simple thing a shot 
thanks for that…any leads on whether the lock cylinder can be replaced ?" 

Given the previous comments, I would suggest that it is not a true lock, in that there are no levers present. There will therefore be no cylinder to replace.