I think the 2072 is a pretty big step up from a 1604. Folks liked those because they were one of the last gear drives CCC/MTD made, but I think for most things folks use garden tractors for, hydro is a better option. The hydro unit in that tractor is more or less bulletproof. Sounds like you need a linkage adjustment/trunion rebuild.
The CH18 is the same as the CH20. you just need to remove the limit screw on the carb to turn it from an 18 into a 20. That might already have been done for you
As much as I love the M20, it's hard to get parts for, and the CH is probably a better long term solution than what was in it originally, as long as the swap was done well. The CH engines have a different driveshaft situation than the M18/20 and KT engines before them. Post some pics so we can see what the guy before you did.
If it's got a 450 blower, you should check to see if the front pulley is the smaller 4ish inch one or the bigger 6ish inch one. Most CH powered tractors had a 6 inch pulley, and that will spin the 450 too fast. The 450 was designed to be used on a 4" pulley PTO tractor (which yours was with the original engine in it). It's honestly not THAT big of a deal as long as the blower is in good shape, but it'll be overdriven by the tractor if it has the bigger PTO in it (and the PO didn't swap the gearboxes on the blower to make it a 451).