That really depends what is wrong with it....
I have repaired two different failure modes....
A lose/wobbly forth wheel on a 6138b. 'Repaired' using a little metal putty to carefully keep the forth wheel pushed evenly off the clutch plate that you get on the 'b' movements. Not a great repair but the only option to me at the time and it worked perfectly.
And if you find that the clutch spring is no longer effective I find it is possible to put a little bend in the spring to make it function once again.