I have no internal flappers left, but in 2019 put a piece of 3" rubber pvc pipe cap over the opening where the exhaust bellows would go (cut with a hacksaw so it's hinged at the top). Note, it's not directly on the pipe, but fitted over a short portion of the exhaust bellows and secured with a hose clamp. It fit perfectly like that, but would be too loose w/o the piece of bellows.
I'm not recommending that, just saying I've got it, and it has worked fine so far. My brother chopped the throttle once not long after I put it on, but w/o any bad results. I wouldn't be too enthused about backing down fast, but that's prob not a good idea anyway.
My long story is that I had an original (batwing) exh manifold overheat due probably to rust restriction. In the process of replacing those with a newer style riser setup, I found that there were no flappers. Now I wondered if there were flapper chunks blocking the exhaust, so I got an inspection camera and inspected, both the U-pipe and the drive as best I could w/o taking it (the drive) off. I found at least one piece of flapper. It was in the drive I think.
I needed to cut my brand new exhaust bellows for some reason, probably to aid that inspection, and I just added the fabbed up flapper mentioned above after that.
It can be done by manipulating the drive (over and up) so you can get in there.
I can see water into the cylinders through the exhaust, but how is it getting on top of the heads? And if it's on top of the heads, how's it not getting into the pan?