You can use Vehicle doc type in ERPNext to set up Vehicle Master. After that you can add this as a link field on your Stock Entry doc type. Set up the items in item master and then you can issue out items from warehouses against vehicles using stock entry with the link field for vehicle.
A simpler way is to create a virtual warehouse for Spare parts and drop all your opening stocks into this warehouse.
After that do a Stock>Stock Entry>Purpose - Material Issue and add all the parts that you are issuing.
You can track the vehicle against which it is getting issued by:
If you want to make one stock entry against each vehicle (Let’s say in a given day you have 3 vehicles that are repaired A, B & C): Here you are making 3 separate entries on ERPNext. You can have a custom field at the Stock Entry level and add the vehicle details in this custom field.
If you want to make one stock entry for a day: Here you are making one entry for all the parts issued to Vehicles A, B & C. You can have a custom field at the Stock Item level and enter the vehicle details into this custom field. If two vehicles use the same part, you will have to make two line items in your stock entry - one for each vehicle. Else you wouldn’t be able to capture both vehicle details for that part.
In both cases you ensure that the expense account you use is something that hits the right account in your chart of accounts.
Of course you may want something more sophisticated to track the entire vehicle check in, triage and maintenance process a custom app on Frappe would serve you best