Replacement Engines For Toro Z4200

My Toro developed an "issue" today while mowing. It began surging with a hammering sound and lost power. I'm going to take it in expecting something inside broke loose. If the shop tells me it's a $500+ repair, I would consider purchasing another engine. Mine is the 19 hp Kohler Courage SV590 series. I can get a replacement for $1000. There are 15 hp models (SV470 series) for $650. My question is whether they are exchangable on the mower.

For cutting around the yard, I wouldn't suffer the loss of 4 hp. If the repair is close to the price of a 15 hp, I'll have it repaired. If it's a grand, I can save a fair amount and do the swap out myself.

Any insight is appreciated.

Thanks,

CR