Having several TVR 1A engines, the last one bought quite recently care of eBay, I found it quite difficult to set up the servo for the forward and reverse control, However, with a lot of trial and error would be solutions I finally cracked it, the solution is as per the photograph, which is to set up the servo, so as the servo arm screw is exactly the same height as the forward / reverse operating rod when level and dead center between forward and reverse, at the midway point, (neutral,) and the ferrule on the servo arm must be allowed to slide freely on the operating rod. This works extremely well and solves the problem.
The solution to the coupling problem is don't use one, fit a fly-wheel to the engine with two contact pins, and a tee bar onto the end of the prop-shaft. After all, you can't have a coupling problem if you don't have a coupling. Regards, Gary.