I was pretty sure that they replaced the props as well (refit episode long deleted) and an article here
seems to gloss over the circumstances of the switch from the 4 to 5 blade prop that was mentioned in this episode.
New engine installation was relatively straight forward as the KTA19 and the QSK19 have similar footprints. The engine beds were beefed up and some alignment issues dealt with. The existing Twin Disc 516 gears are rated for up to 800 HP. The repower increased the output to each shaft from 425 HP to 600 HP. They rebuilt the stuffing boxes and overhauled the 4.5-inch shafts, but the only major change was the addition of new props. “We tried adding pitch to the four-blade props that had been 58 by 43-inch but we still only go 2.6 knots with the engines on idle. So we replaced those props with new five-blade 58 by 55-inch props and this gave us 4.1 knots at idle. At 1500 RPM the new engines give us nine knots where the old engines gave only 7.5 knots. This burns a little more fuel but if we slow to 1360 RPM we get 8.5 knots and save on fuel.”
In the end, if the brothers kind of did this on their own and/or installed it themselves I suppose they are just stuck. I know if i paid for a new engine to be installed in my car and I had to bring it back because the engine/driveshaft/transmission were shaking themselves apart I would expect it to be repaired as part of the initial install.