There are many good choices for D engines. Almost all of the “Italian Stallions” (Rossi P65, OPS, Picco and the very rare CMB free flight engine) will be competive with the Nelson DC 65, which can be hard to come by since only 50 were made.
You can adapt the OS marine to aero use and the K&B .67 with a custon piston/liner is a beast.
All the above will be pretty much equal (although the Picco is heavy) if they are set up the same. If you plan to tip the nitro con hard, you may not want to use a Rossi since they tend to distort liners with high-nitro fuels, but at 15% or less, may be the hottest of the lot. I have been developing them with excellent results for D Speed but that class is limited to 10%N.