Just wondering, is it possible to somehow get a new or an age related plate for a BEC other than using all new parts and spending a fortune or building with all donor parts then converting to Bike engine afterward?

Age related is possible.
It is all based on points.
A chassis from a major manufacturer will give you the biggest chunk. then if you use the donor's steering, brakes and axles, with the correct docs, you will be able to get age related.
Steve,

And I did too, like so

So I can't build my own chassis and be the manufacturer?

I believe, technically no.
However I also believe it's possible to get an age related plate even if the chassis is not counted, but plenty of other "major " parts are.
You'll notioce I used believe twice, here so I don't look any more stupid than usual f someone disaggrees