Many years ago I had a very strange problem with a bike, which turned out to be a damaged crankcase gasket, that was ok when the bike was cold, but when it heated up and the cases moved a little, leaked pressure. Seemed very much like seals, and only found it when I stripped the motor completely.
In terms of CDI boxes, you would probably find that the AC coil/CDI units used on Vespa PE200 scooters would work fine. They can be found on Ebay very cheap, and you should be able to wire up pretty easily, as there is only a trigger, feed, and earth to connect.
Never quite been able to work out why people seem to have so much hassle with CDI units, when cheap and easily available units for AC bikes, will in most cases work on other AC bikes, and only need changes to wiring to fit to bikes.