The valuation form ideally needs to be completed by a garage, dealer, insurance valuer, accesser who should have a fair idea of how much the car is worth.
If you complete the form yourself then it asks for the cost of purchase and the cost of restoration/modifications - This is a good starting place.
The agreed value figure,should reflect the current market value of the car, what you would pay for a standard one in similar condition, mileage plus the receipts you have for the modifications.
Give me a call if you need some help.
Regards
Sarah