It can be but you have to change some things to get it as reliable as a 951 can be. That model is the first with the 951 and Seadoo had to register the engine with the epa so they leaned the carb jetting out. The carbs on it are backwards from all following 951's and have a solenoid that restricts air to the diaphragms further leaning the mixture out.
This problem and the one of the pipe coming loose on the manifold and spraying water out that the carbs suck in must be addressed before any engine has a chance of being reliable. Obviously it will need all the usual things that get done to a sitting ski.... Fuel lines, oil filter, OEM carb kits and needle/seats, OEM jets in your case, battery, fuel valve(possibly), spark plugs, wear ring/pump oil, etc.
SBT has a good price on exchange engines with the best "no fault" warranty. There quality is not as good as OEM but it's what most shops use. I see deals on ebay on NOS parts for those white engines sometimes. That's the good part of having the rare one, when something good pops up you don't have a lot of competition.