I don´t know for sure what temp can reach the zenoha cdi unit, but 90ºc seems high to me, maybe in a extreme condition, running the engine at full throttle all the time.
Don´t worry about the strap ties holding on, they will.
BUT, if you want a better solution, use a width (I think that´s the word) velcro to hold the unit, with a latex foam. The velcro will asborb more engine vibrations than the strap ties. You can attach the velcro with a piece of plywood, screwed to the firewall, on each side. I use this method on all my planes.
Hope you can understand the draw.