Just pour in SGO SAE-90 API-GL4 gear oil in gearbox.
But request mechanic to fill it through proper fill hole instead of speedometer cable hole.
When the gearbox is removed also inspect clutch bearing, clutch and pressure plate and Flywheel bearing and rear main seal of engine. If you have any doubt on these thing's on physical inspection then get them replaced.
If the pipe is in good condition then just get the dholki welded. If pipe is rusted then get the pipe with dholki complete.