I'm not entirely sure what a "power supply conversion kit" is, but if you're looking for an easy way to replace an ARII with a modern switched-mode PS... There are a couple of things about an ARII that would make it a less-than-ideal candidate.
1) It provides audio amplification too (that's the "A" in AR). Off-the-shelf PS units do not do this, so an audio amp would have to be included.
2) Some varieties of the ARII supply some odd-ball voltages (+22 & -22) that few (if any) other PS units generate.
3) Even IF problems (1) and (2) were addressed, and you managed to replace the ARII... many games would still require the transformer block, as it supplies AC for vector PCBs and monitors, and AC for PCBs that have an EEPROM for HS save (and perhaps other things on games I'm not familiar with).
Given all that, most people just fix their ARII boards as they're not very complex and relatively easy to diagnose & repair.
If your main concern is efficiency, the Biggest Loser by far on there is the +5V supply, wasting 20 or 30 Watts. It should be possible to supply the +5VDC from an efficient switcher, but leave the ARII in place for ±22 and audio amplification. This might save you as much as 1/4A of draw from the wall...