Hello T.,
The HM100 is a 10HP rated engine, but an 8HP would still do the job on a log splitter. These were heavy cast iron blocks and held up very well for a lot of years.
A lot of this depends on matching the crankshaft dimensions. Length, diameter and with or without a taper on the output shaft.
The bolt down dimensions may require that new holes be drilled to mount a replacement engine.
Being that old of an engine may take some adapting. I am still a fan of the old B&S flat head engines for applications like this. They were simpler to work on that the Tecumseh and lots of parts are still available for them.
Chinese clone engines are a big gamble and most compatible Honda GX series engines are just too expensive.