To get any help from eBay you must open an item not as described case withinn 30 days from the date of delivery.

Although the default is to return for a full refund, you can if you wish and if the seller agrees negotiate a partial refund. However, there is no provision to freeze the guarantee.

If you miss eBay's time limit but the missing part never arrives then depending upon how you paid you might be able to issue a credit or debit card chargeback, which would have a longer time limit of about 120 days. I think you would probably have to claim for a full refund, or none. I don't know whether the item would have to be returned or who would pay for this.

Frankly, unless this is an irreplaceable, one-off item you might to better to use eBay's MBG to get a full refund and return, then order a replacement. This assumes that the return would be within the UK. Ask again if the return has to be to China, this is too complicated to explain unless needed. (The short answer is that in theory the seller should still pay, but this is uncertain.)