Thanks for the reply.
After some more research I came to the following conclusions:
The fuel mixture gauge will be dropped. My exhaust is too valuable, I'm not going to modify it for sensors. Besides that it's too much effort for information that is not that relevant.
The clock, voltage meter, temp sensor, indicators and speedometer seem manageable to me, although I'm not an expert when it comes to electronics. However, the tacho seems like a real obstacle. I hooked up my multimeter to the signal wire for the tachometer and it seemed to pulse with 1,5 to 1,8 kV or so. Is there a (fairly) simple circuit that can convert this signal into arduino-readable format? or should I go for an alternative like a cable wrapped around my ignition cable?