Go to a machine shop that has worked on 20 valves, and beg then to borrow a tool. You may have to make a tool from scratch. If the legs of a spring compressor won't get down the hole to the spring, then you need to use a different sort. Like a big lever that just pushs down on the spring retainer.
If the head is on the car, you may want to put compressed air down the spark plug hole. I have a hose made up for this, that screws in. make sure both valves are closed. If the piston is at the top, the air will try and push the motor around with the piston at the bottom. So if you want the piston at the top, then put the motor in gear or something once in position. The air is to stop the valves dropping into the motor of course.
I did this job on a Mazda or something a few weeks ago.
Stuff a rag in any oil drain holes, because when the spring collors drop or go flying, they can go down the oil return holes into the sump in some motors, or get stuck half way.