It is very feasible to change the B20 valve seals without removing the head. I'm sure with a little looking you can find the "old time" tools that are made for that purpose. I have a couple of versions in my own tool box. The B20 is easy because there's total clearance to mount a tool.
To do the job--one valve at a time--remove the rocker arm assembly--remove the spark plugs and screw the air coupler adapter (has spark plug threads at one end and an air tool fitting screws onto the other end - any auto parts store sells them)---rotate the crank so the 1st cylinder you're working on has the piston at BDC (bottom--because if you leave the piston at TDC the air pressure when applied will snap it down and fingers could be at risk around the fan & belts)---if you set the crank to TDC for the sake of catching a valve should the pressure fail you need to lock up the flywheel. While the keepers and retainers are in place tap each valve spring retainer from the top with a rubber or plastic mallet. The idea is to shake the keepers up a bit so they'll be loose when the spring gets compressed by whatever tool you use. Keep a pencil magnet handy to snatch the keepers. You'll know the tapping is working -- first "blow" will feel solid -- when the keepers loosen the spring will "boing" and you'll feel it and hear it. Now you can attached the air compressor coupler and go ahead compressing the first valve spring - remove the keepers and retainer--pry off the old seal--slip on the new one--re-install the spring, retainer and keepers release the spring and move to the other valve in that cylinder. Next work on the "mate" cylinder -- 1&4 -- 2&3 so you only have to move the crankshaft once.
The whole project might take a couple of hours. -- Dave