Can't help on the bipe, no first hand experience, also never tried to put a larger Saito in another engine's mount all my airplanes were built for the engine with the engine in hand and never paid attention. Only ones I know about are the Saito 30 is the same bolt pattern as the 20-25 FP's and the 40 is the same as the TT 36.
You can use a plastic spinner and I have airplanes with plastic spinners even though I try to avoid them when I can. Had one come apart on a Saito 40 and sometimes prop clearance can be problematic especially with the Dubro. SIG makes one that they advertise as suitable for APC props and I have had good luck with them.
My preference is the Brodak aluminum spinners although it takes a little work to adapt them to the larger engines. I drill out about half the 1/4-20 threads in the adaptor nut and re-tap it for the engine shaft leaving the front half 1/4 -20 for the spinner screw. You will need to use what is commonly called a bottoming tap to cut the threads all the way to the bottom of the hole. Don't use the large aluminum prop washer that comes with the spinner, install the Saito prop washer and steel nut as normal then replace the lock nut with the spinner adaptor. The Saito shaft is plenty long enough.
If you have access to a lathe and want to get real fancy you can turn a taper on the shoulder of the spinner adaptor so it will fit like the original lock nut. I even go so far as to slot the end basically duplicating the Saito lock nut on the end of the adaptor. Not sure it's really necessary I do it simply because I can.