Actually any modern 25 has a lot more to offer then the old Sportsman 29. It came out about 49/50 along with an 09, 19 and 36. I used
the 09 and 36 in my youth and while some will say that they were wonderful engines, most of my experience was with less then new examples. They did wear out relatively fast but probably more to mishandling and bad fuel then metallurgy. They wern't very powerful a and a lot of us older flyers seem to blot out he bad details of the hobby and remember things in a rosier light.
Usually I had to resort to a shot of 30 wt oil, a prime and then flip the thing fast to get it going. But I was a kid and it was free and life was good, so what did I care.
if your example is in good shape you can use it with an all castor based fuel fo 25/29%. Don't skimp on this or you'll have a nice paperweight and have ruined a piece of modelling history.
dennis