As soon as you wrote this I immediately thought "that's a 7900!". I have one and the only mod is the popular HD filter kit. I gained 10% to 15% more cutting speed just by adding the filter kit and re-tuning the carb. Just with bigger filter and intake that comes with the kit, I'd bet it's near 7.0 hp.
It wears a 20" chain the most and is very easy handling with that. I also have a 28" with full comp and a 37" with skip tooth. Don't know if the 37" needs the skip tooth. It pulls it like a beast through anything.
I know a guy who builds 7900s out of used Makita 6401. He puts on a 7900 head, new 24" bar, new chain, warrantees it for 2 years and sells them for $500. Mines not one of his, I found out about him later.
View attachment 459543