Macca,
Yes you can use a 125cc gasket set on the victa special but you will need a special gasket for the exhaust as well.
As to getting the crankshaft done you will need to find someone that has the big end bearings as well as the crankpin in order to do a full reco and the cost could get as high as around $300.00 +.
We have the equipement to do the job but I would have to trace some of the parts to do it.
Regards,
Bruce