Clutch interchange?
 

I have a 129. Engine runs but is tired. I found what I hope is a better replacement, at least so I can continue to use the 129/ while I do a proper rebuild on its engine, meaning teardown and send thru the machine shop.

I found a much newer appearing, "gray block" K301 in a 127, in the local junkyard. Do "Lew prices" still exist? For what I paid for this engine, I hope so....
the 127 wasn't real nice, but this engine appears to be. I got it for basically scrap price, and I had it started and running an hour after I got it home.
I will have to transplant the carb from the original 129's engine, onto the 127's engine, as the 127's carb definitely needs help, of the kind that I have already given the 129's original carb....

It still has its clutch, all intact etc. Can I use the engine with its existing 127 NF clutch, in my 129 WF chassis, as is?
I know there is a difference between NF and WF mechanical clutches, but I forget where those difference(s) lie.

I "assume" that you are talking about the PTO Clutch. With that in mind, I believe the diameter of the NF & WF PTO clutches are the same. Thus, you should be good to go.

I think the real difference may be later production 129s came out with brake PTO. If yours has a brake, you might encounter issues with the wear button area which probably isn't a big deal.

Mine is a '72 model. It doesn't have a brake. Don't know if it ever did, it was well used when I got it already.

I just ordered Jeff's tool to pull the one from the 129 when I get that one home from the barn and swap engines, so I can get that original engine out, apart and into the machine shop.