It doesn't matter. What you have is fine. I wouldn't have converted your crank to internal balance either. It is very cost ineffective for what you are doing.
If you did want to do that (keeping in mind external balancing is not my favorite thing) you'd need to get a new damper. You can buy a cheap one or an expensive one. Either way it adds to the cost.
Then you have to put slugs of Mallory into the crank to get enough mass in the counter weights on the crank.
If you don't know the reason why your crank is externally balanced is because the cast crank you are using doesn't have enough mass in the counterweights to offset the weight of the piston and rods.
So what is done is extra weight is added to both ends of the crank (or sometimes just one end in certain cases) and that external mass on the ends of the crank add enough mass to get the crank balanced.
That Mallory metal (that's the brand name...it's actually pieces of tungsten) is expensive. That adds to the cost of balancing.
Then you have to install the mallory. Which means you have to take all the bob weights off the crank, take it out of the balancer, put the crank in a fixture so its end is up and you have to drill and ream the hole for the mallory in the face of the counterweight.
Then you press the mallory in and TIG weld it. All labor intensive. And that means $$$$$$$.
Then the crank has to go back on the balancer, all the bobweights installed and you spin the crank again.
If you did your math you should be able to make a couple of corrections and you should be done.
All that adds up to killer costs. It just isn't worth it in a case like yours.
And, I didn't add in the fact that the cost of the reamers for different diameters of mallory.
I don't remember the last customer crank I had to use mallory but I remember mine.
I was running aluminum rods so I would have to drill a bunch of holes in the counterweights to get the crank light enough. That's kind of the opposite issue of external balance.
So I tired of all that drilling and decided I would clean that up by reducing the diameter of the counterweights in the lathe. That would take a bunch of weight off and then balancing would go much quicker.
I decided that if a little off is good, a lot off is KING. I'd just add mallory to make up the difference if I took more off than I needed.
The principle is the smaller the counterweight, the less rotating mass. AT 8500 rpm I wanted light. So I turned the counterweights down until I just touched the rod throws.
I had to add three pieces of mallory in each end. By the time I was done I was into just materials about 200 bucks. At that time a balance job with a clutch was 325 dollars. Now add another 200 for mallory.
If I was charging a customer it would have added another 200 plus another 175 just to install the mallory. At least.
And that was over 20 years ago.
You're fine with what you have.
