How much does the conversion kit cost?

In theory you could run the same piston as a stock 440 but compression will be extremly low.

I am not recomending doing this, just saying it could be done. My lawn mower runs on something like 5.5 to 1 compression. Normal 440 pistons sit at or below deck, the valve notches in a hemi piston do not genneraly extend below deck, normally only for the really big cam ones. If for some crazy reason you were forced to run that rediculously low compression sort of deal an ideal cam would not have valves opening more than a few thousands at TDC.

Again I would never recomend anyone do this, just saying you could, but it will be a very weak motor.