A couple of solutions for you.
1. The 200/225/250 gearbox is the same as the 150/175 except for drive shaft (2cm shorter) and ditto selector. We converted a 175 box to a 200ho gear box last week in about 2 hours. Provided your shaft is OK then it should be no probs if you can get hold of a 175 box. note gear ratios etc are all the same ie. identical box.
2. We have a brand new 250 counter in the workshop so if you get really stuck we could lend you box and you replace it with a new one when it comes in.
We find Evinrude exceptionaly helpful and you should push your dealer to rob a box from an engine. Also if you are highly dependant on the boat for a living then buy a spare, no matter what make you have to start with then problems can always occur and a few days lost trading is covered by a spare !