The parts you mention from the LA 46 will fit the LA 40.

However, the stock venturi supplied with the LA 46-S is almost certainly too big. I would suggest that you start with a stock venturi from the FP 20/25 and now, the LA 25. If that's too small, open the throat up with a 17/64" dia and you will have the same size as the small venturi that used to be supplied with the FP 35/40 in the early days.

I agree with Geoff about the parts you would need. If you over-drill the .25LA venturi, you could put some layers of pantyhose mesh over the top to make it effectively smaller.

In addition, the crankshaft is supposed to fit in .35FP, .40FP, .40LA and .46LA, along with small bits like the prop driver, washer and nut. The case and head are NOT interchangeable between the .40LA and .46LA. The conrod probably works in either, but you should be able to cross-check the pn at Tower's site, along with any other parts you want to investigate.  Steve