On the master cyl. try Rockauto,,,make sure you click on each listing of master cylinder. Sometimes hidden in there you will find a closout on one. I saw one for like 29 bucks last week.
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I haven't done it yet but I asked the same question about replacing the plugs with the heads still on. You can. ( somebody jump in here) Unscrew the cam caps a bit pry out old plugs ,,,wet new caps with silicone ( I'd preferr Toyota FIPG ,a butyl gue, it's thinner and slower to skin up) slide thim in and tighten.
I'd sure clean cap mating surfaces with acetone and put a drop of threadlocker on surface on caps.
I smeared silicone on outside of a leaking CX 500 Honda and it plugged it for a year. but that had absolutely no pressure on it. The valve covers get pretty full of oil when running and there is a little air pressure build up, maybe enough to make it leak more than the honda.
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,,,My crossmember get alot of oil blown back on them , too.
It looks worse than it is. I've polished clean my crossmembers before so I get a good idea of how bad recent buildup of oil and grime is.
I'd slide a full length of card board under there and let the engine run 10 min., and see how much oil has dripped. That might give you a better idea of if the leak has pressure behind it, and the true rate of the leak.