I have oil coting the threads on the spark plugs?
Unfortunately, it's a problem that's only going to get rapidly worse. There are two solutions, and you're not going to like either of them. One, you can have the motor completely rebuilt. That includes re-boring the cylinders and fitting them with new pistons and seals. Two, you can buy a new motor and have it installed. The second is probably the most economical of the two options simply because the replacement, being new from the factory, will require far less maintenance for the first several years.