I used a 12x6 apc for mine. I cant seem to find any of my manuals. I would break it in like any ABC engine. Do not run it too rich, you need the heat to expand everything equally. I like to start it fairly rich and open the throttle all the way using the needle valve to control the rpms. start out leaning with 5 second intervals them richen it to cool it down then lean etc. Do this for a couple tanks leaning it longer intervals. After 3 to 4 tanks you should be leaning it out to close to peak power. Now set the needles with the high still running somewhat rich and fly it slowly leaning it out until its two gallons are through it. The more care you take with breaking in the longer and more consistant she will run. These MDS's are long lasting motors and built like tanks!
The 61 is no longer made replaced by the 68.
Good Luck, Marcel