Here are a few alternatives to that filter, which may or may not be easier to find: Kholer ED0021751850-S / HIFI SO 526 / Cermag 10434
Any chances you could do a remote oil filter and just use a much more common filter instead? The Slanzi DVA1030 engine in my 10kw generator also uses that very same filter, along with a similar style as fuel filter. Incredible expensive and getting kinda hard to get.
Last year, I just built a remote oil filter kit for it and use a much more common and cheap Mann W814/80. The fuel filter was changed to a Mann WK42/2.
This choice of filters was purely based on already using them on my homemade backhoe, so it kinda made sense to keep them the same.