Actually, Yanmar has always made tractors. LOL Mine's a 1981.
YM, F-FX, RS, AF, EF, GC, YT, and many other series in the past 55+ years. All of the John Deere compacts from the late 70s till 2005 were all 100% Yanmar machines in JD green paint.
Should I look for another machine used, it would be the Yanmar/Cub Cadet EX3200 or EX2900.
Should I look new, then yes, the RK37C would be ideal for me.
Yanmar makes many of the engines for commercial boating and pleasure craft globally. Many of these engines in the millions are also used in the Ag machines too. So, there shouldn't any worry of getting parts for any Yanmar engine made for other tractor brands.
Does anyone know how the manufactures also change suppliers too?
Simply put, look at each decade and you'll realize that big brand names keep changing who makes their engines. There are only a handful of engine makers anymore, the field has gotten narrow. Yanmar, Mitsubishi, Kujke and a few others are the makers now in Asia. And China makes the cloned engines. So good, some bad.