Low compression due to leaking gasket in cylinder 1 could be bad enough to prevent firing (diesel needs high compression for the explosion, of course). Yes, you probably do have a leak allowing gases to escape into the cooling system from cylinder 1.
My son's '02 Honda Civic kept blowing out the coolant over time and would begin to overheat. But it was not visible, going out the exhaust I guess. I never saw coolant on the ground. Fortunately, the engine revived with a remanufactured head/new gasket set. Lots of work, but it is fixable.
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-NOS 2020 KTM 250SX (2-stroke motocross)
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