Yes just simply drill out the 4 rivets and install the new flap with 4 new rivets. Super Simple. I have a snow flap ordered from a PRO RMK that I am installing on my D-8 this week when it comes in.
Yes, 3/16 dia rivet. I'd go with a 1/2" grip if you're going up there blind and don't know exactly. Sheet metal is roughly an 1/8" (probably less) and the flap shouldn't be more than 1/4" max. And the "wide" head that was metioned by rmk2112 I'm pretty sure is 5/8" dia.
You might consider going with the OEM replacement rivets. They are usually good quality and painted head if thats whats called for. Then you know you have the right fit. Dealers usually keep lots in stock. Take a look at item #7 HERE for online. Good luck. EW