Well, there are some suggestions above. You can try them. I personally have no experience with an engine without a large area around the cylinder head exposed for the escape of cooling air. I know it's a difficult decision to start cutting away a cowl, and hopefully some of the other suggestions will work. I personally believe that you need to open up the cowl over the cylinder head, but I would wait to hear what others had to say before you begin cutting. It looks to me like you will just get a big zone of stagnant air (which in itself creates permits higher temperatures) in the side of the cowl where the motor head is.

It may be a combination of everything - nearly maxed out needle position, lack of exit for cooling air, and tank being a few inches from the firewall. I have all the same in my Yak, with the exception of increased ventilation, and while the engine doesn't get hot it does sound pretty lean once I get airborne and start wringing it out.

Just my late night opinion