There are a few possible approaches to making a radial based on the Cox 049 engines.
The simplest is to arrange a number of complete Cox engines in a circle, with the crankshafts geared to a central propeller shaft. This would require some off the shelf gears and a couple of mounting plates.
Another possibility would be to make a multi-cavity crankcase, still using the Cox crankshafts and enclosing gears and a common shaft as above.
If you want to make something more like a conventional radial with master and slave rods, you will need to interface the rods to the Cox pistons (swaged joint to a ball end rod) or make your own ringless pistons to fit the Cox cylinders, with very tight tolerances! You will also need to arrange a secondary means to generate the intake charge pressure.
I happen to have a side project ongoing – a 2 stroke radial which uses displacement under each piston seperately to generate induction pressure. I’ll go into more detail on a seperate thread if anything comes of it.
The first two options are relatively simple to manufacture and most likely to result in a running engine.
Anything else is within the realm of experimental and will be a test of patience.