Take my thoughts with a pinch of a salt
It could just be valve stem seals. Not sure how much you know about engines, so apologies if i'm insulting your intelligence here... valve stem seals are accessed by taking the rocker covers off and sit on the top of valves, under the rocker arms. They're a gasket stopping oil from going down along the valve and into the combustion chamber. This could likely be the issue and it means no rebuild require, just a bit of a service The valve stem seals are relatively inexpensive (each one is about the size of the rubber at the end of a pencil) and can be done with the engine in situ, just need a tool that compresses the valve spring, designed for use with the engine in the car. There's a few vids on youtube on how to do the whole thing, as a rough guide - what you do is start with cylinder 1 and take the spark plug out, set cylinder 1 to TDC or BDC (whichever one leaves the springs at minimal compression, can't remember), either use compressed air with an adapter into the the cylinder (this holds the valves up and stops them dropping in when the spring is taken out in the next step) or thread some nylon rope through the spark plug hole which fills up the cylinder to stop the valve falling through. Then you use the tool to compress inlet valve spring (or outlet), undo the nut on the rocker arm, take the rocker arm off, use the tool to decompress the spring completely, take the spring off, slide the valve stem up, quick clean around the valve stem, put on new valve stem, do the same on the so that both valves are done on the cylinder, then put spring back on, compress with tool add nut, set valve lash and when it's all done, remove nylon rope / compressed air, put spark plug back in and then it's all done. Then repeat 7x times. Or alternatively pay someone to do it
But what's more fun than a day of cursing and and oh sh*t moments. Hope that helps!
