The Canadian military used these snow machines back in the day, the hoods were white and they were all season as they were with bogie wheels, I seen one in northern Canada at a airfield and it had a front wheel instead of the single ski and in the summer time they used it to pull the small plains around the airport. Great machines.
150 psi is after a rebuild, any psi over 100 is good. Especially a 640, most likely a little glazed in the sleeves but it should still run like a tank.
Cool video - I'm currently working on my very own 1981 640 that I bought about a month ago, having sat for a good ten years. Got the igntion working two days ago after replacing most of the wiring, and gotta do another carburetor-cleaning as I think the floater needle is still stuck/gummed up and not allowing fuel to travel onwards