I don't know what manufacturers may have used Shibaura engines. Did Shibaura tractors use Shibaura engines or did they source engines from another vendor?
Do you have any equipment repair shops, ex: air compressors, chippers, stump grinders etc near you? Any good repair shop could handle that job.
You could ask your local tractor dealer if they would do the job ever though they don't sell that brand. Many shops are looking for work.