You just need to find the start and endpoints using trig. I and J are the center point relative to the start point. From the little research I just did, arcs are always CCW in a dxf. It looks like you'd take the sine of the start angle times the radius will give you the Y value of the start point (relative to the center of the arc). for the X, take the sine of 90°- start angle times the radius (again relative to the center of the arc). You probably need to convert the angles from (or to) radians.
Your previous line of code needs to get you to the start point. Do the same thing to find the endpoint, which is the X,Y in your G2/G3 line.
I wrote an AutoCAD macro that outputs g-code, but it doesn't do arcs. But It will do G2 and G3 from polyline arcs. A polyline gives you the start and end points, and radius (bulge) but not the center. I find the center using a chord, and the rest is easy.
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Gerry
Mach3 2010 Screenset
http://home.comcast.net/~cncwoodworker/2010.html
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