RE: New Fox engines?
Your biggest problem has been said a few posts up. Lack of demand. I haven't seen a Fox glow engine at an R/C flying field in years. Control line is different as there are some rabid Foxaholics who will defend the Fox 35 to their deaths. It's kind of fun to rattle their cage and let them rant for awhile but it's strictly in a perverted sense of fun.
I've used Fox engines over the past 60 years and once you sort them out they work fine. The problem is that they seem to have so many problems. Especially with heads and carbs. Not to mention the Fox 74 when introduced years ago I carefully broke mine in and was pleased with it until I flew it about a dozen times and the piston holed itself on a less than full power run. Fox explaining cavalierly that they had somehow cast the pistons with too thin a crown and they had a problem. Sure they fixed it for free but it has sat in my drawer going on 20 years with no thought of running it as I moved on to other engines. I can certainly relate other problems over the years but most manufacturers have also had their share of duds. To Fox mfg. credit though they do stand 100% behind their product.