355 Spider seat rebuild

I've found a 355 Spider that I want to get but one problem with it is that there's a huge amount of freeplay in the seat backs. In other words the backrests can move backwards and forwards when they're not supposed to.

I've seen spiders where the seats don't have this problem, but I've been told that it's quite common.

Is it possible to fix the problem by rebuilding the backrest mechanism - if so, how much work (money) does this involve? Alternatively, how much does it cost to just fit new seats?

 

Thanks.