I converted our M817 to a spin-on about 10 years ago, after searching for more than a week for a cartridge.
I use the Baldwin BF-1259. They are around $18, including tax. I think the adapter was around $20. I'm sorry I don't have the part number, like I said, it was 10 years ago.
jwaller is right, they are tiny compared to the original. The flow volume is supposed to be the same, but the surface area of the filter is about 1/3. That means flow will be restricted with only 1/3 the contamination. The cartridge should last roughly 3 times as long, so in the end it might be cheaper.
We keep fresh fuel, and I cleaned out the tank myself, so it hasn't been a problem for us.