I've got one just like your small one, but mine has a 6hp motor. You have to change the belt over onto the pulley beside it to get Hi Lo range. Strong tiller but narrow. I think I got the first Econo horse with 6 hp and then they started building down. My engine is red - troy biilt. Your horse model I thought was an 8 hp, but you have the engine ther and said 6hp. The transmission part of it is pretty rugged. I've rented them over the years. Local rental uses troybilts that they just keep rebuilding.
Good luck with these. Northern tool sell replacement engines I think.
Check out
http://www.jackssmallengines.com/index_model_locator.cfm
Good site for info. The engines should have an ID plate which will tell you the manufacturer and type engine- HM etc. The rest of the machine should have another plate or label - Econo Horse, Horse etc. The engines were manufactuer by different companies - etc. It looks like the horse is a Gardenway Troybilt and maybe the other one is too, or after Troybilt was bought up by some one else.