If you've had a Navomatic working faultlessly for 40 years, very well done! That has not been my experience with them. If you can get it repaired, sure, do that. But bear in mind that avionics have crossed into a new generation (actually, I think that 40 years would be two generations). I have 30 year old King avionics which I bought brand new, and are no longer repairable - at all! So brace yourself that sometimes, a recently, or nearly working system is just not reparable any more.