put in new LLP's and wear components and slap them on. It shouldn't cost more than 2-3 million/engine.
3 million X 4 Engines X 20 Aircraft (spares) = $240 Million, leaving $3/4 Billion for "integration" and "profit".
I can’t tell you why the only engine options on the table seem to be ‘new JT8s or retire the plane.’ But I am sure the engineering studies that would go into other engine alternatives at this late stage would be far too late to meet the need.
And with a a 40 year old airframe, new wings and fusalage/structure repaires will be needed in the not too distant future.