Depending on the nature and location of the crack, it may be possible
to stop drill the crack and seal with Belzona 1111 (Super Metal).
This is an approved method when done properly.
I once fixed an aircooled diesel engine oil pan for a M-60 tank. They
were afraid if the crack was welded that spider cracks would be
created. Also warpage from welding was a major concern.
The US military has Belzona in their BDR kits.