Some on here have had sucess with a left hand drill bit and an easyout to remove the old bolt. It's gotta be on here somewhere.
Mine leaked oil simply from all the off balance vibration of the lose balancer. The oil seal is a simple fix. I alway change a seal when I do the timing belt anyway.
How much oil are you useing?
These engine have a strong bottom end. At 150K you probably have some wear in the valve guides and seals. I wouldn't junk it just for that.

dave h.
'89 Raider V6 5spd;Aisin Hubs;; Gen2 LS: frt. brakes, rear coils;U.C.arms;R.trailing arms;idler arm; rear LSD axle w.disc brakes ;2 battery system for Dog's fan; relocated ECU; custom bumpers;J.Baker receiver;Conferr roofrack; t-bar crank.