You can buy bits here and there (Auto Atlanta, 914 Rubber, Pelican Parts, and more), but 914 Rubber has all of what you are asking about. As far as I know, they are the only place offering complete rubber kits. I'm not sure if their main kit includes absolutely everything, but it is a substantial savings over buying it piecemeal. A top notch business, IMHO.
Pelican Parts has lots of 914 parts, including the battery shelf and support, and will give you a one time 10% new customer discount. They also offer a 10% off once a year discount as a a PCA member, which will likely more than offset the annual membership fee.
I bought my battery tray and support when I was at Restoration Design in Canada last summer. They produce a lot of stuff (as in sheet metal stampings) in-house. Their dedication is proven by their ability to create tooling to replicate an entire Speedster chassis and body! Plus, they use a more rust resistant steel. However, the two parts you mentioned that I bought from them were sourced from a European manufacturer. I suspect the other vendors get their from the same place.
Auto Atlanta has a paper catalog should you get tired of looking at a computer screen. Prices are likely out of date but it has diagrams and numbers, and I’m sure George looked it over closely when it was put together so it should be accurate. I learned a lot from their 700 914 Tech Tips book. Should you find yourself near Atlanta, stop there, and at The Porsche Experience.
My *favorite* place for mechanical parts is 914 LTD in Peoria, IL, because they are nearby (for me), have tons of used parts, unequaled knowledge, and they have answers for all questions. They are just fabulous.
As far as engine holder, this one is highly regarded, but the place is temporarily shut down due to a health issue. I am awaiting their return!
https://www.thesamba.com/vw/classifieds/detail.php?id=606308They make 3,4, and 5 arm versions, are well made in the USA, and still cost less than others. They fit on a 2 3/8” inside diameter HF engine stand tube, and supposedly other, usually cheap, stands. From what I have read, the three arm should be plenty for a 914.
Tom