What can we do / have done to our plane?

[englishal]

What sort of work can we have done to our aeroplane without loads of EASA or CAA paperwork or approval costs (and time)? EASA CofA, was CAA Private Cat.

For example, if the AI or DI needs replacing, are we allowed to just source a replacement and have it fitted - even if it is not exactly the same model? Can we have fitted a FS-450 fuel flow computer by just buying one online and giving it to our maintenance organisation and asking them to fit it?

Secondly our engine instrumentation is rather basic - although it works I couldn't tell from looking at the oil pressure gauge whether the oil pressure is slightly different from yesterday, or the oil temperature is slightly abnormal. Ideally I'd like to fit replacement gauges as a back up, for example something like this:

http://www.umainstruments.com/images/special/3in1.jpg

So I can see on a decent scale how the engine is performing.

I am guessing that this would be a major mod of sorts and hence prohibitively expensive for a not every expensive, simple VFR aeroplane?

Thanks...