Rob:
I do not know of any I2C RTC chips that come completely bundled with batttery and crystal.

However if you are having issues with calibration you should look into the M41T00 chip from ST Micro. This 8 pin RTC has a really cool calibration system built into it that is setup such that the chip can speed up or slow down the 1Hz generated to the seconds counter by very small amounts by skipping or adding clock cycles coming from the 32KHz crystal.

This is a very good way because it permits calibration from mucking directly with the 32KHz circuit which by definition will change its characteristics when you try to probe it or adjust it. With this ST chip you can calibrate either by hooking a 10 or 12 digit counter to an Fout pin on the part that is buffered from the 32KHz. You read out error and compute the correction factor which is written into a couple of registers in the RTC chip. The other way is to derive error by accumulating operation over a long time frame and compare start time to end time against some accurate reference such as WWV or NIST. That error is then factored back into the part in the same way.

Hope this helps
Michael Karas