I know several of our clients have had Honda 4 stroke 130s fitted to Lencraft Ribs in the 6.5 to 7.5 recently and seem to be fairly happy . I'll make a few calls tomorrow for you and get more accurate info .
300kg sounds very heavy given that the Yammy 115 4 stroke is only around 170kg
Two of my Army Boat Squad RIBs are Lencraft Lynx 6.8 m boats fitted with Yammaha 115 2 strokes and both pull 35 knots with a fully armed 4 divers and Cox aboard
I hear whispers that Yamaha and Suzuki are close to realeasing 200 hp 4 strokes this season .
On the subject of where to buy your engine , my advice is always if possible to buy it and have it hung by the guys who built your boat .
Best wishes ,