Easier to put the '37 powerhead on the 39 driveshaft housing and change the model number tag. Many differences were made. Different exhaust pipe length, different drive shaft , etc.. Anyone would (should) know it would be worth less money than an original '37. For general motor use, it would be more dependable. The piston pump wore rapidly running in silty conditions. The later models are much preferred for digging channels through sand bars . . .
Living my dream because Reality is too expensive . . .
Alan, my propeller guy fixes scags all the time. Last one he fixed for me, I made a pattern from another motor out of cardboard. Came out real good. Some people are masters with those helearc welders! Jim's Prop Shop Standish MI.