As for my thoughts or opinion:
As most of you know, I'm a collector and restorer, so I would want to rebuild the original motor, or like Doc said above, try to find an exact replacement of the same year, manufacturer, horse power, and spec number, especially if the rest of the tractor is in great shape overall. I too try to keep these old tractors running with original parts and pieces, so that the History of the tractor can be correct and accurate. I would rather wait 10 years in trying to find the original part, rather than settling for a reproduction part just to get by. That is what makes this hobby fun, and it gives great joy to find a part that you have been waiting so long for.
I had a situation once, where I thought I bought something kind of rare, only to find out later that it wasn't. I was at a public sale early in my collecting years, and on the auction block was a 1966 round fender, that had a HH100 Tecumseh motor and 112 hood decals. Since there were only 1,000 112's manufactured in 1966, I thought I found a gold mine. I ended up winning the tractor for a couple hundred dollars, and thinking that I practically stole the tractor since it's considered rare. Long story short, when I got home and was able to spend more time in looking over the tractor, I did some owners manual research and found out by the serial tag that the tractor was actually a 1966 110, and should have had a K-181S 8hp Kohler in it. Someone along the way repowered this tractor with the 10hp Tecumseh and installed new 112 decals. As you can imagine, it was a big disappointment, and then I realized I spent more money on the tractor than what I should have, as at that time, the 1966 110's were pretty plentiful.
In this case, who would be the blame for this mishap? I was partly at fault, as I was new to collecting at that time, and didn't know how to read the serial number tag to see if the tractor really was authentic. I didn't take the time to look the tractor over real well at the time of the auction, as I didn't want other bidders to know that I was interested in buying the tractor. And, I wasn't experienced enough to separate my emotions from my wallet, and let the thrill of owning a somewhat rare tractor get the best of me. The owner who repowered the tractor is also at fault, since it appears that the tractor was being pawned off as something that it wasn't. He could have easily made an announcement before the auctioneer started, that the tractor was repowered, and the motor and hood decals weren't original for the tractor type.
I guess what I'm pointing out is, is this. If you have a nice original tractor that needs to be repowered, but you decide that you don't want to, or can't afford to repower the tractor with an exact replacement, then please keep some type of paperwork trail on the tractor explaining that the motor has been changed, altered, modified, replaced, or was increased in horse power, so that others will know what was done. If something should happen to you tomorrow, and your wife decides to sell off your collection, she isn't going to know what changes you made, and will be a little embarrassed when she tries to sell your tractors as complete originals, when they aren't.