57 35 hp seahorse

I am new here. Thanks for any help in advance.

I picked up two older Johnson outboards this past November
One is a 1955 25 hp all original that runs like new. I had to replace original fuel lines and water pump but other than that it is a fantastic running and looking motor. I put it on an old 1961 Lonestar 16' boat. Looks great!

The other is a 1957 35 hp seahorse with electric start. A real rough looking motor that needs a lot of work.
I picked up a replacement starter because the original didn't crank over the engine. It engages for a second then stops. Unfortunately, the replacement starter does the same thing.
With the plugs out,it turns over great. Plugs in, turns over for 2+3 seconds.
If I open the compression bar on the back, it turns over.
Could it be the starter needing new brushes?

Compression seems very good,

Any help or suggestions are appreciated.

Well, in the first place, those starters were adequate but not overly so. The whole system must be top notch for it to crank properly. That starts with the battery and then the cables. Especially the cables connections with the battery. They must be shiny clean and tight. Moving on, the cables themselves must be adequate. Skinny substitutes or jumpers don't get it. Now to the other ends of the cables. Again, clean and tight. Don't assume, look and deal with them. Starter solenoid is next. Lots of solenoids get replaced needlessly. If it works, it probably is ok. Now to the motor. Same thing where cable plugs into the side. Clean contacts are a concern. Remenber, the screw that holds the cable to the motor is the ground connection. Don't ignore it.

Now that you have checked the basics, if it still doesn't crank, do some voltage drop tests. If you don't know how to do this, either get some help, or learn how. Stuff CAN be tested. Replacing parts without knowing what is wrong is foolish.