Re: single speed chain issues
Have you tried removing the chain from the engine drive side, tensioning the bike drive properly, and then reapplying the engine drive chain and playing with the provided adjusting tensioner?
You should be able to get both tensioned properly without a chain tensioner on the bike drive chain. You can buy half links from certain websites if necessary.
If you want a tensioner I'm sure there are some options out there. Only thing is most tensioners are made to bolt onto a derailleur hanger, I belive. Most bikes with horizontal dropouts lack these.