A circle viewed from from the side is an ellipse.
A common approximation can be found on the web (eg do a google image search for isometric circle). This produces something like (with arc centers T,U,A and N):
Alternatively, make a 4 Bezier Curves with control points of each arc being the mid points of a side of the parallelogram and the included corner.
Here is the ellipse which is the image of circle: $x^2+3y^2=3/2$.
Here is a picture of the ellipse (black), the composite Bezier curve(red/green) and the 4 circular curves(blue/orange):
How is this justified? Why do the arcs join up smoothly?
EDIT: I would like to:
Predict the error (for a bezier approximation to a circle here is an error estimate: wikipedia).
Generalise either of these methods, say, include more Bezier control points, include more composite arcs, find more circular arc centers ...