From Oddyssey batteries:
Define the difference between MCA and CCA.
The marine cranking ampere (MCA) rating of a battery is very similar to the CCA rating; the only difference is that while the CCA is measured at a temperature of 0°F, the MCA is measured at 32°F. All other requirements are the same — the ampere draw is for 30 seconds and the end of discharge voltage in both cases is 1.20 volts per cell.
I don't think there is an exact conversion but 850 would be fine.