Engine life
The TBO of the O-235 is 2400 hours and you might be able to extend the life by 20% (but don't count on it).
The engine overhaul should cost about £8-10,000 if none of the major parts are scrapped, you will have to add another £3-4000. for fitting and other parts. You can cheapskate but you are unwise if you don't get the engine frame crack checked and re-painted, fit new hoses and fit a new oil cooler at this stage.
Most Lycoming engines have a life of about TBO 2000 hours for normal use with aerobatic engines having a TBO of about 1600 hours.