1991 11000 HSC Fiesta BIGG oil leak

I recently purchased a 91 fiesta with a 1100 engine, it has a serious oil leak. I have cleaned down the engine and isolated the leak to either the sump seal at the gearbox end of the engine or the crank main seal at that end of the crank. Having not worked on many fords I would like to know whether these engines are suseptable to crank seal leaks (a big job)or whether it is more likely to be the sump seal (a little job). Thanks for any info
Tony

not an easy job to do the crank seal,have you checked the engine is not overfilled,and that the engine breathers are clear?