Can explain a liitle further please. Is this the lever in teh back-rest or the lever on the seat hwere it pivots? I have my 400 seats dismantled at the moment and might be able to make some suggestions or supply some digital photos of the mechanism so others can comment. Unless you mean the plastic trim is broken, the actual mechanism is pretty simple and the seat itself is easy to strip down.
1977 400 Auto
1976 365 Boxer