Hi GTX and Paulie,
I'd bet London to a brick that the current power plant is a 5/8 shaft unit and the only reason there is a new clutch half fitted is because the original cotter equipped unit had been allowed to run loose for quite some period of time and thus became flogged out, in turn this destroyed the rails and why they have had the repairs done as we see them currently.
I'll be the first to say that the older style Briggs and Rattler engines were the most unbalanced units around and thus this was the start of all SB Twin Rail fatigue problems on the 45. The newer style engines are far more vibration friendly on the Series 2 Chassis and thus I'd strongly recommend that if you were considering a replacement that you would choose one of the newer style sloper designed engines.
The current 3.5Hp Briggs units are pretty good but if you can afford a Honda then go for it.
The current Briggs units are now being sold for around $250, whilist Chonda's (Chinese Hondas) can be had for around the $125 mark delivered to your door.
Horses for courses I guess. Choose your Horse wisely.
Cheers,
BB.
