Hi Ron,
You're very welcome. It's fun sharing knowledge & experiences in this hobby isn't it?
I am currently running a 20x8 prop on my OS GF40 (40cc 4-stroke gas engine). When I was flying this on the electric system, I was using an 18x8. Performance in terms of thrust was quite good with the electric 18x8. I think your Syssa should have no trouble running these props from what I've read. I loved the sound of this 4-stroke engine and compared to many other 2-stroke gas engines, it is relatively lightweight. When I converted mine to gas power, this removed the lipos & speed control & speaker system from up front and so to balance the aircraft with the OS GF40, I put approximately 1lb of lead & epoxy in the inside of the nose cap. Again, total aircraft weight was approximately the same at the end of the day; I just had to shift things around to get it to rebalance. Your Syssa is even lighter than the GF40, so it seems like a great option for this pusher.
I run a 16 oz tank mounted approximately where the CG is, in the area behind the pilot (which incidentally is where the fuel tank was on the full scale DH2). You will want to make that turtle deck removeable so that you can easily access the fuel tank, plumbing & anything else you put in there. In addition to the fuel system, I have my ignition module & ignition battery mounted in this area. My turtle deck is removable and I'm very glad it is since this has made maintenance much easier.
If you don't have a cordless battery box for your starter, I suggest getting one. One of my initial thoughts on the advantage of going electric was that starting a gas engine from within the tail booms & support wires would be risky from a safety standpoint. However, in practice there is (just) enough room and it helps if the starter is cordless.
best regards,
Greg