RE: Maximum altitude??
You can workaround it by having a servo operating the mixture needle.
However, this is "blacksmithing" at best and definetly not keeping it simple, if that is an aim.
Servo operated mixture needle requires you to adjust the mixture in-flight, and you have to guess what needle position is required for which altitude.
Even worse still, you also need to guess the altitude of the aircraft unless you got telemetry with real time feedback.
And no model IC motor manufacturer will provide you any chart for altitude vs mixture, hence you need to work out on your own how to adjust the motor for which altitude.
In my opinion, that is a university project of its own.
And since propulsion is not the main study of this particular project, I'd say its just not worth it to do it more complicated than necessary.
If IC is even a realistic option, i'd recommend turboprop.
Gas turbines is by nature much less sensitive to both mixture and thin air.
My first choice would still be electric, because its simplest to operate, most reliable and most versatile.
Possibly also cheapest, given the trouble with making a piston IC work with altitude.