So I can not say much about that model of 750 as I own an earlier model, but I have always thought that Honda has built a good motorcycle. As to the value that depends on where you live basically and what work has been done. Sometimes work that has been done can lower the value. When it comes to...
My guess is that the gasket didn't seal on the top end or if it is coming out of the port then possibly the valve isn't closing all the way or the seals aren't working.You will most likely be taking at least the exhaust off to verify a little bit more of the problem.
I agree with dirtdigger, I have tore apart many sohc engines and there are only the ones in the parts diagram. In fact I have never seen that part you pictured any where in or on the engine
It's to bad you didn't check for crank movement while you had it apart. It's possible that the bearings are bad. The other thing that comes to mind is it's possible that transmission is sticking and creating the tension. I would take the top end off and then check the movement.
Had that problem on one of my bikes and I ended up restricting the air flow by adding a filter type material in the pod and it fixed the problem. Not sure if it will work for you but might and any suggestions are better than none.
If the stem is the right length and all you need is the bearings I would take measurements of the stem where they mount and do the same for the frame. Then contact a bearing manufacturer. I think you should be able to find something to fit