My 05 900 is my daily driver, can anyone help me troubleshoot a coolant leak?

So I have lived in a very unique place on top of a ski resort for the last two years. My daily commute to and from work at the base area often includes some combination of a chairlift, skiing/snowboarding, hiking/snowshoeing, or my personal favorite a snowmobile ride on the 900 I bought from the previous tenant. I have some mechanical knowledge but was completely new to 2 strokes at the time.

At least, it was my favorite. Then I snapped my steering post this winter season. I wound up taking it to a nearby power-sports mechanic for the replacement I was hoping to save myself time and stress, as my winter months are often non-stop working, playing, and being a general ski bum dirtbag. I've got enough mechanic experience to have done a top end on a 4 cyl Accord a year prior, so I probably should have done it myself.

Takes the guys longer than I thought, but I got the sled back with a few new problems I hadn't been having before. Starting, flooding, and bogging down issues all seemed to be fixed by replacement of the plugs. But there is still an awful coolant leak, that I think is entirely an over pressurization issue causing coolant to be force out my overflow hose.

Coolant is dumping out the overflow hose at an alarming rate. Basically as soon as I start pulling the starter. It can easily burn through all visible coolant in the tank in a single commute to work, about 2km. After it will hiss and steam and smoke, but not smell like a typical overheat, more like it's just steaming coolant off hot parts. Haven't checked the actual temp. Haven't checked the compression.



So my thoughts;
A. Coolant Cap
B. Thermostat busted or incorrectly installed
C. Mechanic failed to bleed the air properly
D. O-Rings (hope not as I'd have to be stuck walking home until summer most likely [then in the summer I have to walk home anyway cause no dirtbike yet]))

That's all I really got, the actual coolant bottle appears fine. My first inclination is to replace the cap, check the 'stat, and bleed it good this week cause it's cheap and go from there. Anything else I could be forgetting or ideas on how to test this? I also notice my TP boots are cracked, could the be messing up the fuel mixture and causing it to run lean? I think that could cause the pressure build too.

Keep in mind that prior to the steering post snapping, these problems were nonexistent. Topped off coolant only once since I got it a year and half earlier. This problem came about immediately after the engine was most likely pulled and replaced (steering post is in the back). This makes me think that it's probably air in the system or an improperly installed thermostat. Leaning more towards air cause I "THINK" that coolant is actually cycling.

All that being said I really am in the dark with 2 strokes here so any thoughts from you guys? Any other info I can provide?

Thanks, I think I got the overflow under control. Thermostat was fine and I bled the system and it's mostly gravy. I think I nice new tight timing belt would help with the pull starting, it seems really loose. Had it running and no leak last night, got the track unfroze and right as it started to spin it died. Hard and quick stop. Pull starting after that felt really off, and I checked the belt and had like 3in of play. It was late and like 9F degrees so I came inside, but gonna play with that today. If not it's probably my recoil

Thing broke dow right before a few nice storms too. So I had to get off work hike up about a mile and dig it out from under a lot of snow to even tinker on it. It's buried in a nice snow pit and if I can get it moved by Friday neighborhood is gonna tow it out with one of the cats.

I need to get this thing dialed in. It's a lot easier to invite company over from the local bar if I don't have to make them hike. Come this summer I think I'll work on a rebuild either way, but I just want to move it by end of day.