Chances of a datsun engine being stripped to measure cubic displacement woul be 1 in a million.
You'd have other issues to worry about before the power of an a14 brought you onto trouble with s company that insures 1000hp skylines and such openly.

Also insurance is usually based on duty of disclosure. You state when your doing the quote as well as by the policy that all the modifications on your car are legal and rwc. Reason why everyone's not wanting to touch it is because it may not be lgal and rwc as you have advised them.

In some cases it's actually condoned by road transport to stamp your old number onto a replacement engine that bares no such number. It has happened with older cars that have had replacement engines, if it's missing then it may be allowed to restamp it If approved by road transport auth. They do it with vic/chasis numbers some times too.