Advice needed. A journal cap came off!

Sorry gentleman but if that were my engine, or a friends asking for advice, I'd do a total dismantle. You have no idea why the cap came loose in the first place, yes could be torque and it could be bad threads hell it could be a lot of things. You maybe able to reset that bearing cap back in the block but a runout should be taken and also checked for egg. At the very least the crank needs to be removed and checked for straightness and the journal repaired as required. Also, you'd never catch me changing just one main bearing, even on my lawn mower, ****you replace one main bearing you replace them all****.