I believe you'll need to remove the cylinder head to do that. Don't think you can remove the spring and seal without the valve falling into the cylinder
That means you'll need lots of work and a Toyota Repair manual.
You'll need to change head gasket once you lift the head.
I'll say try to get a used head, get a shop to P&P it, clean it up with new seal and shims. Swap the ported one in and sell the old one.
You may want to do a full rebuild also.