You can make any substitution you want. You won't get the beer you're trying to recreate, though. I'm a huge MO fan, and to me it's well worth the extra few cents per pound. If using actual MO is not an option for you, you can approximate the taste by replacing it with a mix of 95% 2-row and 5% victory. I highly recommend splurging for the Maris Otter, though, especially if you're making a malt-forward style.