MAS is primarily a squadron management game. Your primary objective is manage your resources so that you have the means (aircraft, ordnance and pilots) to complete missions, which gets you more money, etc. There is also a light roleplaying element as your pilots gain experience, you choose where to improve their skills.
There is a combat game within the system, but at best, I'd call it "combat light". It abstracted sufficiently that most engagements are resolved in minutes and an entire mission within 30 minutes (assuming all are present). There is a engagement map that shows relative positions, and miniatures could be used in place of tokens to show what planes are where, but it is by no means a miniatures game nor is it a detailed combat sim; I'd call it more of a narrative combat system.
That said, it is our opinion that it does a pretty good job of capturing the "feel" of aerial combat, and to a certain extent, ground attack, as well.
As for conversions, that's a bit tricky. Jay handled the stats for MAS, but I can speak for the WWII conversions. Some of the stats are objective, such as altitude, range, ordnance limits, and so forth. Others, like maneuverability, are far more subjective. In both cases, planes are compared to each other to help determine some of those stats.
Because of the interrelational aspect of the conversions, it's not like VBAM where you could theoretically have ships from anywhere in the same game. Because the stat range is limited (generally -2 to +3), the planes from the WWII expansion are not at compatible with the planes from the modern base MAS set.
Now to be honest, I found it's not that hard to setup a conversion. For the WWII stuff, I built a spreadsheet and plugged in real numbers for stuff like number built, max altitude, speed, etc. I used Avalon Hill's Air Force for the more subjective stats like Climb, Dive, Maneuver, Evasion, Damage Modifier, and so forth. Then when I had most of the planes I wanted to include and could find all the necessary info for, I started breaking real stats into ranges to determine where the cutoffs would be for the various altitude bands, speed modifiers, and so on. Whenever another plane needs to be added that gives me the guidelines on how it fits in.
For the purposes of another system to run Crimson Skies dogfights, all I can say is maybe. It depends on whether you want to make all the crunchy decisions, the MAS or MAS-WWII would not fit the bill. If you aren't as concerned with the crunchy details but still want some decision making with a fair potential for excitement, then the MAS system might suit your needs.
Be sure to look at the various promo sheets that are over on the website. That should at least give you a feel for the level of detail (or lack thereof) in the MAS system. And certainly feel free to post any more questions you might have!
