This is a similar situation to what I am in with my engine, but I started calculating before doing work on the head (work should commence shortly
).
There is some info on my calcs here
http://www.ausmini.com/forums/viewtopic ... sc&start=0 about 2/3 of the way down the page.
Here is some more info - I have the same size engine (1100+60), but am going for 10:1 compression. I did some of my calcs assuming a piston - deck clearance of 50 thou. This clearance can be reduced by decking the block, but I added it in some of my calcs to get an idea of how accurate they are and how much room I have for error (especially since I took an educated guess where I didn't know some of the values - piston dish, ring land, gasket etc). I followed the calculation method on page 222/223 of Vizard's yellow book.
1st calc:
with 8cc dish, 0.7cc ring land, 3cc gasket and 50 thou piston to deck clearance (4.35cc), I calculated the combustion chamber volume required for 10:1 compression to be ~16cc. The same calc with no piston dish gives ~24cc. The same calc with no piston dish, and pistons flush with block gives ~28cc.
My second round of calcs was to see the effect of skimming a head by 60 thou, based on what I read here:
http://www.ausmini.com/forums/viewtopic ... sion+ratio
(I haven't checked the accuracy of what the Doc said here, just using it for roughish calcs to work out what I will have to do to get my compression ratio around 10:1. Will fine tune the compression ratio later)
With a 1098cc and a 998 cooper head, compression should be about 8:1 (see link above), which gives a total combustion volume (including gasket, dish etc.) of 39.3cc. A skim of about 60 thou gives a compression ratio of about 8.5:1 (see link again), which gives a total combustion volume of 36.7cc. This means the combustion volume has reduced by (39.3-36.7=) 2.6cc for a 60 thou head skim. This obviously depends on chamber shape as well, so it is only a ballpark figure.
The starting size for a 12G295 head combustion chamber is 28.3cc. Assuming a 2cc skimming reduction will give 26.3cc. This is enough for an engine with no piston dish and pistons flush at TDC which requires 28cc(see earlier calcs). It also isn't too bad for an engine with flat tops and pistons 50 thou down the bore, which needs 24cc. It will give around 9.4:1 compression in an engine like this. It won't really suit an engine with dished pistons though - even with pistons flush and a dish of 5cc it will only give 9.2:1 compression.
This last set of calcs are all assuming no head mods on a 12G295 chamber. They may be close for your head, but you really need to cc the head to work it out (if you are modding the head you need to do this anyway)
Re: your options - I spoke to a guy at work a while back about machining back pistons and block to reduce dish and he said he doesn't think it would be worth it. It only costs about $200 for a new set of pistons (finding a set that fits your engine shouldn't be a problem, but if the engine hasn't been freshly bored it may be worth boring it to clean up the bores. If it was freshly rebored in this build then its all good.) Then it comes down to about $200 for new pistons and rings Vs about $300 (I think) for a 1275 cylinder head. I am going for the flat top pistons for my engine because I already have rockers, valve retainers etc to suit the head I have and can't be bothered with the hassle of finding another head and getting it to where I am (1600km from Perth) and then stuffing around getting retainers and everything. Easier just to swap the pistons and keep everything else.
