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Good luck with your build! The stock Zuma crank is tougher than people give it credit for, I'm only familiar with the bugeye and horizontal engine though. I blew the motor on my 70cc setup, which is why I decided for a new engine. I ran the stock crank on a high tuned homeade pipe, it peaked at about 10,500 rpm, but i tried to keep it below that. The failure was in my cylinder, I added 2 auxillary exauhst ports on an Athena cylinder, the total exhaust port was close to 100% bore width. One of the bridges between the main exh port and the aux port failed, and turned the piston and cylinder into scrap. I did split the cases to install new crank bearings, but that was probably unnessecary, and it was a major pain. I had the case halves on the grill and the crank in the freezer. I think a lower tuned 50cc or 70cc tune is great, very reliable and still very fun.
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