There's no crew as yet. I have some Peter Pig German artillerymen on the way, which I hope will do the job.
The fighting compartment and gun conversion piece is available for sale at http://shpws.me/OMkj.
I've created a sprue of three of this model and uploaded it to Shapeways.
It can be bought at http://shpws.me/OPHQ for only a little more than the cost of a single piece, since most of the cost of most prints (in the sizes I'm working in) consists mostly of Shapeways' handling fees rather than the print resin itself.