Invest in a stethoscope and have a poke about around the chain area before you buy a new motor.
A cam chain will rattle more than knock.
If you have one, and if you can, to eliminate a big end knock have the bike running and plugged into a computer, now with it going tok tok tok tok, retard the timing bit by bit until the noise goes.
If the noise goes, it’s the big end, if not… Keep looking.
This is assuming you can adjust the timing with it running.
Or, get a manual chain adjuster and slowly wind it in and see if the noise goes.
If not, big end may be toast.
Or… Check the oil pressure, find what it should be and compare the two amounts.
Excessively worn bearings will drop the oil pressure by quite a lot.
I’d hate to buy another engine for no reason.