As said previously, take the head off then whack the 2 highest pistons, dont worry if you damage them as you will need to replace them after the rebore.
I would always rebuild an engine in preference to throwing in an unknown one.
youll need
Full engine gasket set ?10.30
4 oversize pistons with rings and little end bearings ?12.34 each
set of mains ?4.43, big end ?10.50, and thrusts ?1.86 (get the crank measured to see if it needs grinding)
Oil pump ?13.80
Timing chain, tensioner and giudes. ?6.19
Check the cam and followers but they will be ok.
service kit (oil, plugs etc.)
even new engine mounts are only ?1.36
replace all the radiator hoses ??
Rebuild the head when the block n crank are away for machining
inlet valves ?1.43
Exhaust valves ?2.26
Valve guides ?1.49
so for ?120 + vat and a few days work you will have a belter of an engine.
Hope this helps.
Drew.
(The above prices are from Paddocks.)