I just found this on the net it states that there are some problems with valve seat arrangements that have been known to cause issues. You might like to think about using the updated heads that they mention. just a thought!
The first models had some problems with warping valve seats, so later cylinder heads were changed to round exhaust ports to help the heat dissipate. Heat remained a problem, frequently causing the head to fracture. The balancer and camchain arrangements needed regular attention.31 Aug 2015
Not saying this is the problem but valve seals are a source of smoke if they are old or worn plus valve seats do wear and that might be an easy fix!! Press out the valve seats and put in new round ones plus seals.