Just a suggestion. You might do some research on school bus diesel engine and transmission combination's and find something that would fit your need for a conversion.
School Buses are cheap when sold from school districts and many have engines and transmissions in excellent condition. Also you would get all related equipment including running gear, air brakes etc if you wanted them.
I know in some parts of the USA, buses are used to make specialized equipment because they are so well maintained and easy to get. I might add they are cheap. (from 1,000 - 2500 for the latest ones)
Just my two cents worth and not worth that to a lot of people.