Luc
I went through the same exercise ....
I am thinking your J has a serial number somewhere between 24-0024 and 24-1037
the lens is made of plastic and is Mooney part 202816. If you have a parts catalog, it will be in section 57-20-00.
You will not find a Whelen part number beacause Wheelen does not make that part. The Wheelen STC that for the A650 system includes all similar "flat wing tip" models but does not include the J. I will let you decide why it is that way. Only Mooney makes it. Please, don't pass out at the price for a small piece of plastic.
You would nee a field approval to install the A650 system.
If you look at early Js, you will see that their lens is always opaque, ocre looking and cracked. If / when you change the lens, it will become opaque very quickly ... fun ...!!! So might as well order a few in advance.
I saw once a Mooney that has that exact lens made of glass and I wish the owner was near so I could ask him how did he get a glass lens replacement. I was never able to find one.
Sorry for using a bit sarcastic tone today.