I dont want to appear negative from a legal perspective the reason why the police are usually so quick to dismiss any jurisdiction here is the fact that you have (a) paid this guy for services and (b) handed equipment to him to work on.
If he has not given firm turnaround times to complete the work, then that works more in his favour.
As does the fact that you voluntarily gave him the equipment.
Im sorry but 90% of the time, the Police wont be interested for these reasons - unless enough people lay complaints with them, they may feel so inclined to take some sort of action.
But overall, its a civil issue. Even if he gave you times to complete the work, there isnt really an industry yardstick in which to measure when the reasonable tech person will have work complete and the unit back to you by - this guy can keep saying "Im working on it" for five years, and how would you convince a Judge (who knows nothing about repairs) that this is BS, and that effectively, he has committed conversion?
The best ways to try to get recourse here are (a) keep nagging the guy for the return of your amp & money (b) turn up on his doorstep to try to achieve same, (c) think of the how many people you are scaring off from using this guy when this thread pops up on internet searches (d) use whatever financial avenues you can to recover the money (if possible) - ie: paypal, your bank, etc.
Quite honestly, Im surprised this guy keeps using the same name - its pretty easy to change your name to an AKA, I thought. So he cant be that smart.