Rivet size for the 'Vespa' script on a GS 160

Hello All:  I just bought a restored '64 GS 160 and am thrilled.  It is black (I know, I know......). The 'Vespa GS' script on the front of the leg shields is black and thus invisible.  I want to get one of the SIP ones in 'chrome-look' (assuming they are recommended, look good, etc.).  Here's the thing:  The SIP web site says that the rivet kit for this is 4X6mm.  I've just measured the rivets on the present script and they are 3 mm across the head. 4mm strikes me as pretty hefty.  SIP has another kit which sells a full set of  2X4mm but I'm wondering if they are too small.  Will they expand by 1 mm once in place?  Seems like a lot! Thoughts....?

Finally: I'm pretty handy, but have never riveted before.  Do I really need to buy a 'riveting kit' too?