1981 380 SEL sucked the transmission filter?

[MAF]

This car sat, inoperative,for 12 years, before I bought it.137,577 on miles, replaced fuel filter, tank screen, and pump.Started up, and ran.Went home(on trailer),changed oil, and began to gently drive around.Ran great, would not pass emissions.Took it to a local shop,(good /reputable), and on the way there, it would not take off at a light.Gave it some gas, rap, rap, bang, dropped in and burned rubber, like I was dropping the clutch.
Got to the shop, they drop the pan,(on the trans), and it has sucked filter material,(decomposition, due to sitting so long?), into the transmission.
They changed fluid, and filter, slipping in all gears.I wondered if it was just in the valve body, and had them switch out , to a different,(verified good), valve body.No joy!!
Would a power backflush have a chance of clearing out the filter material?
It ran great , apparently before sucking filter.
What to do?