Although I am not new to rc aircraft the problem I recently encountered is new to me. In a new build, the Hanger9 Tiger 30cc, after completion of engine I realized the spinner supplied with the kit won't fit the size prop needed for the engine I'm using DLE35RA. This engine is an optional power plant recommended for this plane. Prop required for this engine is 18 - 20 inch. The spinner openings are way too small for this application. I probably could carve out a bigger opening but I am worried about throwing out the balance of the spinner and compromising the structural integrity. Apparently the spinner hub size is determined by 1/5 of the prop length. The spinner hub that came with the kit is 2 3/4, which, if I'm correct, swings a 13 in prop. The prop I am using for this engine swings a 19 in prop, so it will need a spinner diameter of over 3 in! The spinner that comes with the kit, at 2 3/4 dia fits nicely about 1/8 smaller diameter than the opening of the cowl. The wooden prop I'm using is 3/4 in thick. Just want to get this right.

I would absolutely never use a plastic spinner on a gas engine. I’ve always run aluminum spinners or carbon fiber. That said, it’s pretty normal to need to open the prop. A google search on 3” aluminum spinner brings up lots of options.

One source that showed up is Tru-Turn. IIRC, that company closed down over a year ago. I would recommend CALLING any of the companies you want to work with to make sure they are still alive and well before placing an order.

True-Turn has not closed down. They have stopped manufacturing spinners. The parent company is still very much alive and last time I contacted them is still selling remaining inventory. IMO still may be worth a phone call. That said, still other options available. Some spinners being made come drilled for a couple different engines. Odds are that if you order a size appropriate spinner that comes pre drilled it will fit both 30cc and 35cc.

There are lots of very good quality alu spinners from China on eBay. I bought a 3,5" alu spinner a month ago and got it in two weeks (to me in Sweden).

You apparently didn't read my last post, beyond skimming it. I said "One source that showed up is Tru-Turn. IIRC, that company closed down over a year ago. I would recommend CALLING any of the companies you want to work with to make sure they are still alive and well before placing an order". I didn't say the parent company was done, just that Tru-Turn was done. This was something that had been reported right here in the forums.
Here's some of their website's information page, last updated July 2022:
Tru-Turn is suspending manufacturing in consumer model hobbies as of April 2022.
To the Weekenders, Competition Pilots, Builders, and Designers throughout the World – we thank you all for allowing us to offer our high precision American workmanship to this fantastic group of people sharing a common interest in flight.
• ROMCO has ceased manufacturing of consumer level components.
• We will continue to offer available inventory & modifications through Jun 30 2022.
• Items available after June 30th will be offered from finished parts inventory only while supplies last.
• ROMCO must focus all manufacturing capacities on domestic Industrial, Aerospace, Commercial UAV and Military production.

You claimed they are out of business, they are not. They have shifted their business. My comment was that they are still selling remaining inventory. Perhaps you should stay in your lane.

I am staying in my lane. Since YOU don't have a monopoly on aviation, or information based thereof, I will post anything I deem appropriate

Sure thing, just keep posting misinformation. That you seem to have the monopoly on.