Will lawyers handle a civil case involving 21000 in damages ,,,...for a set fee as opposed to retainer plus hourly?

It's very unlikely that an established attorney would handle this type of case for a set fee, because the attorney has no control over how much time he/she willll have to put in.  It may be that a settlement can be reached right away, or the defendant may insist on taking numerous depositions and making numerous motions, all of which the attorney would have to spend time on.  It's even less likely in a $20,000 case, since the set fee would have to be pretty low.  You might have a better chance if the contract required the loser to pay the winner's attorneys' fees, and the fee could be a set amount plus whatever the court awards as attorneys' fees.  Then if the defendant drags the case out, the lawyer could at least have a chance of recovering for the extra time.