Rose of Sharon: How to reproduce ASAP

I've got a purple flowering Rose of Sharon thats almost 50 years old. It has 3 main trunks coming up for the central point. Two of those are held together with bolts, the third has had its bark sheared off and exposed for the first near 2 feet. One of the bolted together sections has half of it dead, I could rip it right off. It has some green moss-like growth on it. It is not in the worst shape, but surely could be better. It still blooms nicely, purple.
Nearby there are two new ROSs that have planted themselves are have been growing for around 7 years. They are taller than I am now, 6' and would be suitable replacements, but they are not accessible.
I have just researched today what do to try and create a replacement for it. I learned that the shriveled up flowery enclosures that fall off in the fall have seeds inside (brownish) that apparently are the seeds for newer plants. But I also read something about taking a cutting from the lower part of the tree and growing that in a plant. And I read something about the seeds not reproducing into a true representation of the original.
I want to get a replacement, to take out the old and replace with a new, using the one that I have, in as little as time as possible. What can I do. What size container do I need, and if I use a smallish one, will that stunt the growth of the plant when I eventually transplant it into the ground in the same place as the one I want to replace? I want to do the replacing in as short a time as possible, getting the sibling as large as possible, as soon as possible.
I don't know what to do to find the seeds, what to do with them, or what to do with a cutting. Any advice?
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