Detonation info: http://www.2strokeheads.com/tech-octane-detonation.htm
I have no idea if your compression ratio is off as I cannot measure it. I'm also in New Zealand so I cannot tell you about octane as our measuring system is slightly different to yours. Lots of things can cause detonation, its just that your motor has recently been worked on so I would check the compression ratio first as it is relatively easy to do. I'm just trying to point you in the right direction... but all the answers can be found on this site and others like macdizzy with a butload of reading.