Rinse it a couple times with fresh gas , dump those out and fill with fresh gas , shouldn't be a problem.
Need to know what what engine your working on.
you have 2 sets of bearings. there are main bearings and rod bearings. they are not what you call hard to put in but you have to know what your doing and have a torque wrench.
The crankshaft turns in bearings. To replace them you have to disassemble the engine. If the crankshaft is worn a machine shop might have to grind it for undersized bearings.
There are different kinds of bearings in the engine, including main and rod bearings. Depending on which you need to change, it can be difficult or relatively simple. Main bearings are the easiest to change, you can access them by removing the oil pan. Take off each main cap and you'll be able to remove the upper and lower half of each bearing. Make sure to clean and inspect the surfaces for damage, and clean them really well. Lubricate the inner surfaces of the bearings after installing them with assembly lube. Be sure to torque the caps back on to the right spec. Hope this helps!
