Froglick, I see you're from south central KY, so if you haven't jumped on something yet, you should be close to Barlows in Somerset. Maybe you could get into a diesel ZTR and not have to look back. I hear that Barlows is good to deal with.
Mitsubishi MTE2000D, Dig It 258 Mini Ex, Deere Z930A ZTR
Ouch, those prices are outrageous. The prices I found were considerably less, they were doing it yourself or course. A shortblock is about $1500, A 20 HP Honda conversion is about $1500 and a 18 HP Briggs conversion will run about $1400. Any of these would allow you to keep the mower for considerably less than any of the figures mentioned before and the engine swaps are very straight forward Saturday projects. I have pulled the engine out of a Grasshopper 1622 and it was a very simple job to take out/put back in...maybe a 3-4 hr job with my teenage son lending a hand at times. The shortblock just requires your old heads, carb and muffler and is a little more work but still an easy weekend deal. Grass Hopper 618 Outdoor Power Equipment Repower from Small Engine Warehouse
I think a Command swap could be possible for much less too. The difference between the Triad crankshaft and the Command series is that the Triad 's shaft center sits 5.25" above the engine mounts and the Commands shaft center is 6.15". Grasshopper has obviously used the Command in other versions of the 618 mower and likely simply just used different motor mounts to account for the crank height and a 1" bore clutch ( this is just my speculation). The difference in crank shafts between the Triads 618's and Command 618 is yours has a 1 1/8 x 3 1/4" crank shaft. I guarantee if you are handy enough to fabricate engine mounts that are .9" lower and find any Command engine with a 1 1/8" x 3 1/4" crank, you can make it work....it may require simple electrical issues, muffler, etc and it takes work. It's also delving into "no mans land" and generally isn't for the squeamish. I probably shouldn't have even brought this up but I just went through a kind of similar experience with a John Deere zero turn and saved a ton of money on an engine swap I figured out.
Unfortunately, the Kohler Triad TH engine stands for Triad Horizontal (shaft). The Grasshoppers all use horizontal shaft engines but most of the Kohlers they used were the Command series. He got great service out of his Triad, but it's a bit rare in comparison.