I guess it's a little late for this post, you have a lot of time and money invested but I have to say this. I sure hope you did a compression/leak down test on that motor before you started down the road you have been on. It sure would be sickening to learn you have put all the work and money into a motor that's used up. If not, you might want to do one just to see. If I remember right, that was still when they were running 85 to 90 psi with a good, charged battery and spinning at over 250 rpm. Low compression is going to kill hole shot.
That's not a problem if it will let you take all the spark plugs out to do it but a lot of times the starter won't let you do that. The Bendix kicks out when it hits the cylinder with the compression gauge in it. If it does that, you have to do it one plug at the time. You also need to do this on a warmed-up engine. Let it run on a hose for at least five to ten minutes, then do the compression test. A cold engine will lie to you big time.
I prefer the leak down test but if doing a compression test, I usually let it hit on each cylinder six times. The main indicator is that all cylinders are within 5% of each other, and no more than 10%. For an accurate test, you want the same number of pulses on each cylinder, I've found six to be a good number.
I don't see you doing the leak down because the gauge is expensive, you need 100psi air pressure and it's a little harder to do, but if you have to try that, you still don't want more than that same 5% to 10% max.
A DIY rebuild on a four cylinder is probably somewhere between $800 and $1,000 at today's prices for gaskets, seals, pistons and rings, and boring and having the block and heads resurfaced. Most of the time, bearings are reusable, other than the lower crank main. This is also something I would not recommend unless you are very familiar with the internal working of that engine. Getting the rod caps back on perfectly is an art without a special tool that holds them in alignment.
So, I suspect a DIY rebuild is beyond your abilities and having it done or buying another power head leaves a lot of room to get shafted.
A couple of weeks ago, a good friend of mine had a good friend of his call me about a V-6 Looper lower unit, and to help him out I sold him an extra one I had (cheap). When the guy picked it up, he was telling me his mechanic that was very good with these things had rebuilt his with new gears and all, and when he picked it up, he noticed he could shake the prop shaft around in it. The guy tells him it would be fine, that was normal. The next day, he goes out and all the gear oil had leaked out in the driveway, so he was hesitant to use it. I'm thinking, and this is a mechanic he thinks really knows a lot about them. RRiiiigghht! I told him to try and shake this one around, if he could, I would give it to him, naturally he couldn't get any movement out of it.