Following up on Chris Mullet's suggestion, did you try blowing air into the tank through the pick up tube point? If the repair shop was sloppy, they may have got tank sealer into the pick up tube. If that happened I suspect you are semi screwed! I cleaned and sealed the tank on my 1971 142 E. As the sealer was setting up I was regularly blowing compressed air through all the fittings on the tank to keep them clear. Once the sealer has set up it is rock hard and exceedingly difficult to remove.

Whether the pick up tube has been plugged up or the end is rusted off or it has holes in it I think your only option is remove and repair. It has been 15 years since I worked on my tank; but, my recollection is that the pick up tube fits into a round fitting on the side of the tank. Pick up tube on the inside of the fitting and a threaded port to accept a tube fitting for the gas line on the outside. My recollection is that circular fitting is soldered to the side of the tank. If I am correct, you should be able to unsolder the fitting allowing extraction of the pick up tube. You could then remove the pick up tube from the fitting, fabricate and attach a new pick up tube to the fitting (likely have to swedge it) and then solder everything back in place.

I would be inclined to go back to repair shop first to see if they are prepared to fix the problem.