Came across this ad and shocked my memory into remembering I had this model when I was much younger... Tempted to get one again. Has terrible reviews on Amazon for lousy quality control.

Visible Rotary Engine Plastic Model Engine Kit 1/5 Scale #11201 by Minicraft (11201)

 

Have one; got 90% through assembling it, and the little spur gear that presses onto the motor shaft to make it work shattered instead of pressing on.

Also, it's a 10A.

 

Believe it or not, Fleet Farm sells a clear plastic model piston engine! My awesome grandma got me one for Christmas a few years ago and I thought it would be the coolest thing ever. Then I couldn't even put one of the pistons in the cylinder head because they hadn't shaved enough material out of one of the cylinder walls. So that got returned and I got a pair of chopper mittens with the money from returning the display.

Big side note there, but I always have wanted a rotary see-through display like that.

 

For $55 you can almost buy a REAL rotary engine for display, lol