CHECK CHECK CHECK!!!!
Before lifting the head either take off the whole rocker shaft assembly or remove all 8 pushrods. This causes all the valves to try to CLOSE.
Get a long straightedge, and sit it on the valve stems, to see if any of them sit visibly LOWER than the others. If so, cry.
Now get a big socket on the crankshaft pulley and turn the engine over. Feel for COMPRESSION on each cylinder.
You can now check for undamaged valves by running the starter motor - as the engine turns over you will (should!) hear a regular fast-slow-fast-slow .... beat as the pistons hit compression in turn. If the beat is obviously irregular you probably have one or more bent valves, which means HEAD OFF.
But if all 4 cylinders have good compression, try fitting eight straight pushrods a new timing belt and tensioner, and some words of prayer.
Worked for our Def90, total bill £39 for bits!
CharlesY