We sold Pentons back in the 70's . when the first 400's came along the were called Mint 400 after a race we had in the States at that time. The power delivery on the first one's we got was like pulling the trigger on a gun chuckle chuckle!!!!!.
Riding one in the woods on one track with handle bar width trees about was down right scary !!!!!!
In the early 90's we found a 79 GS and restored it. It was a different bike from the first one.
That generation engine was a good one. True ISDT bike. Would last a long time and put out good power.
Here is a picture of that bike.
We add one or 2 things to it but it was fun to ride in the woods. They had changed the porting and pipe design , added some weight to the crank and a heavy motoplat flywheel. It would pull from the bottom up nice and smooth. Had long legs and would handle well.
If you can find one like it i think its a very good choice for vintage off road.