The 35s had a beefed up lower unit so the gears could take a bit more stress, but the lower unit from the 30 HP will probably be OK as long as it doesn’t get abused a lot.
Dave

Do not know where you live salt water area or freshwater but eBay always seems to have 30 HP. power heads on there pretty regular. I have a Johnson 30 that had a seized power head and although I am capable to rebuild it it was much more cost effective to just get a freshwater good used power head. I live in Ohio which is the mother load of old outboards. Our boating season is only about 6 months long and most old motors were not used too hard. Michigan, Wisconsin, Ohio, Pennsylvania are states with plentiful old outboard engines. There are other good states to get freshwater parts from also but the motors there are just not as plentiful. I bought my complete replacement power head off eBay from Kansas it was from a Javelin 30 HP. the guy just parted it out to get rid of it. Bought it for $75.00 plus shipping. It was in beautiful mechanical condition and obviously very low hours of use . I could not have been happier. All I had to do was change ”O” rings in bottom seal, new top crankshaft seal and a head gasket which really did not need to be changed . ( I removed head to inspect cylinders ). The motor when assembled ran beautifully my favorite outboard to use. My point here is that if you or anybody is redoing a 1950’s OMC 5, 51/2, 7 1/2, 10, 15, 18, 25 , 30 or 35 and the power head is totally seized and rusted it is much cheaper and sometimes better to just get a good used correct year power head for $50.00 to $100.00. My seized original power head would have needed everything pistons, rings, cylinder boring, bearings, gaskets, etc $$$$$$$ of dollars. My tips for getting a good used power head . 1 get one from a freshwater state. 2 look at the pictures of all the bolts and screws on power head if paint is not chipped off good sign nobody has screwed with it internally. 3 If bought off eBay look at the sellers other items if they are selling a power head most of the time they will have the other parts of engine for sale and you can get an idea of general condition over all. 4 read feedback of sellers past sales if they have 100% good feedback and good feed back comments your probably good. 5 ask them to please double box it with extra padding between boxes they get throw ed around in shipping. Sorry to be long winded but but I see a lot of people here with seized engines wanting to buy a ton of hard to get parts to get there engine going and even then probably not even using it 15 hours a year. You my friend I would look for a 1956 30HP. power head from a Johnson , Javelin . Evinrude or Evinrude lark. Why build a Frankenstein motor when you can have a correct year motor? Hope this helps others enjoy this hobby. Bill,