Mercury 4hp 2 stroke - No spark

Hi All,
I'm hoping an electrical guru on here can give me a few ponters.
Purchased the above engine a while back as a non-runner project to re-build, it was quite ropey and the original problem the previous owner had was that it would run for about 20 minutes then cut out and not restart.
To cut a long story short, mechanically it's now a beauty, re-built her, all electrical components removed, mating faces clean as a whistle, tested the spark post re-build, it was a strong blue spark that easily jumped 1/4"
Put her in the tank, ran her up, started and run just fine, after about 20 mins, right enough, dead cut, tested the ignition and there was no spark.
That was a few days ago so there's nothing related to an electrical component overheating.
I've been troublshooting (resistance only, waiting for a DVA adapter to arrive) and found the following
Capacitor charging coil - Bad 61.1 Ohms - Manual calls out 93-142
Trigger coil - Good 92.5 Ohms, Manual calls out 80-115
Primary coil - Good 0.4 Ohms, Manual calls out 0.02-0.38 but I should think 0.4 is close enough
Secondary coil - Bad 5.6 K Ohms, Manual calls out 3-4.4 K
So the questions are, can anyone give any reason the engine would be fine and produce a spark after sat there for a year or so and then just have a total spark failure ?
Given that the Charge coil and secondary windings are both out of limits, do I need to go out and buy both before I proceed any further with troubleshooting or are there any further test I can carry out on those components.
Many Thanks. Tony

Serial number? Single or twin cylinder?

Sorry Fitz, don't have a serial number, its a single cylinder.

I'm working from the 4/5/102cc Sailpower manaual which covers serial numbers 0A809601 and Newer (USA & Canada) and 09477830 & Newer (Belgium). I'll bet its a Belgium engine.

I've done some futher work. Problem solved . The Tohatsu service manual gives better testing procedures and DVA readings.
Charge coil had low resistance initially and on the DVA was outputting 33 DVA where the manual calls for 100 DVA minmum, I'm guessing the engine was just about running with the spark avaialble and then as soon as the coil warmed it the resiatance lowered further until it simply wasn't kicking out enough to charge the CDI. Replaced the charge coil, all good, just run her for 30 minutes and didn't miss a beat.