Crank case welding.

[b1k3rdude]

My worry is if it's welded will the pump still be central and will it ever be oil tight again  I guess :/

If an engineering shop did the welding they would/should then mill out the excess etc to give you a perfect whole. If the replacement engine has lower mileag, in better condition, dosent need anything major doing, comes with the usual 1 month warranty and you have the cash then its the simplest fix.