The HK P-38 flies beautifully, so it was an easy choice to sink $35 into a decent set of 32g servoless retracts. The stock retracts don't quite work, but at $140 for a twin motor PNP P-38, it's hard to complain. Still, I had to glue the stock retracts in the down position to keep them from collapsing when approaching a pebble.
This mod is fairly easy. The new retractable nose gear comes with it's own strut. I used the stock gear struts for the mains. The stock wheels and tires work fine but I added 3 silver-hubbed P-51 wheels with rubber tires.
I cleared foam to the base of the nose gear well, then mounted the retract to a piece of thin CA'd plywood. The existing nose wheel steering system converts with min modification. The retractable mains mount inside of the original gear anchors, sandwiched and screwed into two .25x.25x1" inch CA'd plywood blocks.
Total mod time took about an hour.