Ski actually looks pretty good.
I'd just focus on seeing what you have and get it running if possible. Pull the engine if you haven't already and find out what it's condition REALLY is. It may be cheaper than you think to get it running. A 550 is going to be plenty of fun as is. Rebuild the engine and put new turf on it, a set of grips and RIDE it. Don't let it turn into an unfinished project you will sell to someone else some day. You want to actually ride this thing right?
Be aware that you will need to carefully align engine, drive shaft, and impeller if you remove engine or pump. It's not simple but it's not difficult either, there is plenty of info on how to do it.
I have a 440 only slightly modified (after market exhaust, bars, impeller and extended ride plate) and ride it twice per week, still not bored with it! Just picked up a second 440 with a 550 top end, so yeah they are fun.