How to forecast common sub-assemblies used across multiple parent items?

I am in a situation where our client is a manufacturing company which produces machines. Each parent machine is unique in nature. So, always the new item master/branch record is created for the parent machines.

But most of the sub-assemblies and few purchase parts are common across all the parent machines.

The requirement is, how can we calculate the forecast for these common subassemblies/purchase parts?

I was thinking of using forecast generation for parent machines and try to blow down the demand for components/subassemblies, but for that we need sales order history. As we always create new item number for all parent machines because of its uniqueness, we will never have sales order history in JDE.