Ive successfully welded two crankcases myself using TIG welding process, but i must warn that its not without risk due to:
1) The aluminium quality can be variable,
2) Porosity in the aluminium, and
3)Contamination with oil,
Each of the above factors makes the task challenging and i think that you’ll struggle to find a welder who will accept the associated risks due to the potential destruction of the crankcase, that was what i found and is why i tooled up to do it myself.
If you would like me to have a look at yours send me some photos and i’ll take a look and advise,