Tevita Li was his usual handful on the wing, while Damian McKenzie scored a nice try off the bench.
In the wet conditions, New Zealand's set-piece looked far sounder, despite Scotland's scrum prowess, but Boyd was realistic in stating that more work was needed in this area to trouble South Africa.
Renton, who made nine appearances for Otago last season, relished his first start of the Junior World Championship.
"It was good to get the opportunity. I thought the boys got a bit of go-forward, and we looked good when we hung onto the ball," said Renton, whose first try came when he latched onto a loose Scotland lineout tap-back to scoot 20m for the try and the second was even more opportunistic, after some footwork by his teammates on a breakout. He was happy to be in the right place at the right time.
New Zealand were in full control into the second spell, when the weather was at its best, and, after two scratchy second halves in the first two games, the brains trust will be more content at this area.
Captain Simon Hickey limped off with injuries to both his ankles, though he should have come off earlier.
Today will give a clearer picture as to his availability for Sunday's showdown. There were no other injuries to report.
The two sides are staying in the same central Auckland hotel, and while Boyd said that the respective managements got on well together, he joked there may be some "frosty" encounters in the lobby or lift over the next few days.
Countdown
JWC playoffs (Sunday)
(Albany)
South Africa (1) v New Zealand (4), 7.35pm
England (2) v Ireland (3), 5.05pm
Australia (5) v Samoa (8), 2.35pm
(Pukekohe)
Wales (6) v France (7), 5.35pm
Italy (9) v Scotland (12), 3.05pm
Argentina (10) v Fiji (11), 12.35pm