The 54" was not recommended on that model. The biggest recommended deck was a 48". I would think the 16HP would be underpowered for that large of a deck. Therefore, the engine should be fully checked out since it was possibly working it's tail off to drive that deck.
Find out the service history, also. Those engines were supposed to have the valves adjusted every three hundred hours. I will tell you from mine, it does make a big difference in how they run. It can also make a difference in the engine life span.
I bought a 1641 new in 1992 with 54" deck. I mow 1 1/2 acres. I have never add any problems with engine. It burns no oil and i have mowed grass as high as 6 inches - with deck at full height first pass. It is still running strong today!
Thanks for the helpful information everyone. I thought about it some more and I have decided I am going to stick with my 108 for now and see what I can do to make that work much better. I know this will be helpful for others if they are considering at 1641 in the future.
I picked up a 1641 last year, two Engine mounts were missing and other 2 were so loose motor pulled out of the coupler and quit so the guy ( a mechanic ) sent it down the road. i bolted the engine back down and its a great runner.