As Don said, undersized bearings will say so on the back. However, I
would stop short assuming an undersized bearing means a properly sized
crank. I have seen and heard of some pretty ignorant practices used to
keep our little British cars running.
Pull the pan and the rod caps. Look at the bearings and the journals.
If you see copper on the bearings they are shot. Run your fingernail
across the journal. If it catches on any grooves then you should, at
the least polish the journals or, preferably, have them ground. If one
or more of the bearings are obviously damaged, then you should pull the
engine anyways because you now have bearing shrapnel in the oil
passageways and the engine will probably self destruct shortly after
putting her back together. If the journals are fairly smooth and all
bearings show copper then you are dealing with everyday wear and just
replacing the shells will probably work. Buy new shells of the same
size as those removed. Also buy some Plastigage to check the clearance.
Plastigage is used with the bearing shell assembled dry but torqued to
standard settings. Compare measured clearance to the factory spec. A
thousandth or so extra is OK for a driver but you will run lower than
expected oil pressure and could starve your valve train of oil it the
engine idles a long time while hot.
The main bearings are usually OK but, if they are easy to replace, go
for it. Some engines are notorious for eating thrust bearings so it may
not hurt to replace these while you are in there. I have never seen the
inside of any of the engines used in Morgans so I can't provide guidance
here.
Good luck,
Bill
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