Here is a good analysis that is a bit dated but should be relevant.
The data is from 1985-1995, 160,000 planes and 20,000,000 flight hours
Pilot related: 80%
The focus is mainly on pilot error (which is an interesting read). The breakdown of the mechanical/maintenance issues is as follows:
Oil system: 5%
Fuel System: 3%
Vacuum Sys/Instruments: 2%
The NTSB has a lot of data available either in reports or by database query. The reports only break mechanical issues down to powerplant/non-powerplant. During 2007-2009 it was about 80% powerplant, 20% non-powerplant for personal flying. Other types of flying had various distributions. If you consider oil and fuel as powerplant systems, then it's about the same as the data above.
Anecdotally, from reading through many GA accident reports where maintenance was a factor, a lot of things come up after a maintenance visit. Things not attached properly/securely, not done correctly. So having good maintenance, and doing a very thorough check afterwards, will help.