The simplest solution, which will solve your problem positively in 15 minutes, is to replace the head pump unit. It is about $95 and will basically give you a hew head with an old seat. Really easy.
Not sayin' there aren't better heads. There are. But this is an easy good fix that was not mentioned. Should hold you several years and very hard to do wrong or to get in trouble.
Working on a head does not need to be a messy job, if you flush multiple times first. Really. Same thing with replacing the head. Flush, flush with vinegar and soak, and then flush once more. Easy.