As others have said, the length of your chain run is a large part of the issue. That, and a seriously unbalanced weight distribution. You have an awful lot of engine on a very minimalist bike. I'm a little concerned with what you said about the engines origins, as well. Typically, vertical shaft engines don't do well when run horizontally - the lubrication systems usually are not designed to operate in that position.
You asked me about your power side chain issues, and I can easily understand that you may be having such.
First, the plane of rotation of the drive sprocket needs to be absolutely parallel with and co-planar with the plane of rotation of the driven wheel mounted sprocket. The best tool there is to establish that is a key chain laser, in my experience.
Second, the length of chain is a big factor in the amount of stretch the chain will exhibit under load - the more links, the more stretch. Check chain tension constantly.
Third, your idler on the return side is set to a sharp angle and closer to the sprocket than would be best. I'd suggest you take a straight edge and place it in contact with the underside of both sprockets, and then mark on your seat post the height a straight line chain run would cross it at. Remove the existing idler, and mount a spring loaded arm with idler cog (not a smooth idler wheel) to raise that bottom chain run at that point about one inch above straight.