Revisiting this
Hs 129 was a result of specification that required a small 2-engined aircraft, powered by 'non-strategic' engines (ie. by the engines not required for the 1st line A/C), well armored, and with some meaningful firepower.
(the non-strategic engine part was flawed IMO - once you must manufacture two engines per A/C, and the total HP provided is lower than on a single 'normal' engine, while needing two propellers, two oil systems, that math falls in the water, but I digress)
Let's change the spec a bit, and apply it for other countries, too. Still a small 2-engined A/C is required, well armored, with very good firepower, preferably 1-seater, engines in question are preferred to be of non-strategic type, but without going into extremes so the AC is under-powered, IOW no need to go for 400-600 HP engines. Good guns' firepower is needed, so is the carriage of a lot of small bombs. No bomb bay is required.
Yes, some air forces don't have a thing for tactical A/C that much, let's have that changed for the purposes of this thread.
Aircraft needs to be in service by early 1940 in it's 1st version.
What is non-strategic varied a bit from country to country in 1938-40.
For instance for the Japanese the Nakajima Ha1 Kotobuki or around 700hp was mostly definitely a strategic engine at this time.
Italy is in the neighboring boat, if your strategic engines are around 800-840hp what are the non-strategic engines?
For some of the other countries engines existed or were advertised but the abilit to build large numbers may have been lacking?
For France the Gnome-Rhone company offered both 7 and 9 cylinder radials of 350-500hp and 600-700hp but since they used the same cylinder assemblies as the 14 cylinder engines actual production without cutting into the 14 cylinder production may have been limited. The 9K engine was sort of a 2 valve Mercury.
Or production capability cutting into the 14M and 14N.
HS had the X series engine of around 700hp but how many they could get while trying to build as many 12Y engines as they could get?
Lorraine may have been on their way out. There was 300-500hp 9 cylinder radial of 20.8liters. They advertised larger engines but actual use?
Getting into the Renault air cooled V-12 here, There really wasn't a lot else.
Germany has the two 9 cylinder radial already discussed
Italy has the air cooled Isotta V-12s (two sizes) and a few 7 and 9 cylinder radials of 700hp and under.
Soviets have...............not a lot that doesn't impact 1st class production unless you take used engines off old aircraft.
Trying to make a 9 cylinder engine out of M-88 parts might work but then you have sort of a Mercury engine but you are cutting into M-88 production.
British have similar problems, after you go lower than Mercury engines (being build in a shadow factory for Blenheims) you wind up with AS Cheetahs or DH V-12s of about 500hp.
The US isn't much better. Only P&W and Wright are building radials of over 300-350hp. and Wright has a gap between the Whirlwind 9 (R-975) and the R-1820. Now Wright does have several different R-1820s in production at the same time. Not sure how the production is handled. Production of the "G" series continued on for a while but mostly as replacement engines for existing aircraft. G-100s and G-200s are front line engines.
P&W has the R-1340 (500-600hp) to go below the R-1830. They have or want to stop R-1535 production.
There is a gap in many countries between the 500hp and under and the 850hp and above engines. The 700hp G-R 14M was something of a rarity.
Guns can be another post.