you can try brazing or oxy-acetyline welding...actually, it might be safer to use mapp/oxygen or propane/oxygen.
if you've got access to a welder w/ the spool gun and a tank of argon then you can def try that, but make absolutely sure that the sealing surface where it meets the other case half is completely flush. otherwise your halves won't meet properly and you'll have a leak.
i'd def recommend splitting the cases and welding the piece apart, but if you decide to just weld the hole w/ the engine assembled then take note that you'll have a hell of a time when you need to replace the crank or crank bearings (very common) or rod bearing or connecting rod. or if your balancer bearing wears out and cuts into the case which is also pretty common.
again, all reasons why it's in your best interest to pull the motor and split the cases.