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Is the engine in there got something wrong with it? Just sitting usually doesn't hurt them or the transmission. IF you do change it out, you would be best to put back the same engine and transmission it has now.........or you will have to do a lot of modifcations. Those engine and transmissions are in thousands of RVs with good results. Better to perhaps just rebuild these if you think they prove out bad and add some modifications for better fuel mileage and shifting performance. Lots of possibilities there. At least do some preliminary testing on those to see if they are still good or not. You might just have to chean up the fuel system and filters. Please get back to me if you need more on this or need to discuss other upgrades. All follow ups are included in our fee.