Specialist Classic car insurers’ will provide what you need, I think it is called “Laid up” cover.
You can also an “Agreed Value” as opposed to “Market Value” reflecting the cars real value and the policy generally covers spare parts as well.
Once you put the car back on the road you can obtain cheaper classic insurance based on an agreed mileage limit again with an agreed value.
If you look in classic car magazines there are always plenty of companies looking for business, mine was with Footman James.