Welcome to the forum! First off, I should preface by telling you I only have experience with the GTX 951 models, but there are many others on here that have vastly more knowledge on other models than I do. That being said, I can tell you what I have learned from personal experience.
As far as the speedo, check the paddle wheel mounted to the ride plate on the left side of the pump. Does it spin freely? Is it broken off anywhere? If all checks out back there, double check the wiring on the inside of the ski. The harness should be zip tied to the black electrical box on the port side. Make sure that 1) it's plugged in, and 2) there isn't any corrosion on the connectors. Whenever I unplug something or do any electrical work, I use a little dielectric grease on the plug to help keep water and corrosion away.
If taken care of, this is a great ski to ride! Part of that involves fogging it. I have heard that to prolong crankshaft bearing life, fogging every ride is the way to go. Moisture tends to build up on the insides of the DI machines, and coating the parts in oil will help to prevent that. I'd say it's less of an issue if you're riding it multiple times a week, but for a few times of usage a month, I'd probably do it when you put it away each time. It adds a couple minutes, but it's a lot easier than replacing an engine!
I'm sure you already know this, but if you flip it, pay attention to the correct "roll over" direction as indicated on the sticker on the stern of the ski, always use a synthetic API-TC oil (preferably BRP XPS), keep the battery fully charged when stored to prolong the life of the rectifier, and limit out of water running to a maximum of ten seconds or so.
Most of all, have fun with it!!
