if it is a crg heron “plus” it is an early 2000 model. heron plus had a yellow chassis with black components and was the first round of calipers that led to todays awesome crg brakes. at that time there was a love hate with the brakes….they tend to lock up if you dont have a very sensitive feel for them. i loved them but most thought they were too sensitive. there were three models which had no markings to tell them apart from what i know. the way you can tell is by where the main side rail attaches to the “u” shaped front half chassis rail. the plate that holds the brake master cylinder is what helps indentify soft, medium, or hard/rain chassis. the longer the plate the softer the chassis. the longer the plate from the side rail the softer the chassis because the rear main rails angle in at a faster rate and weld further back on the ‘u” section.
hope this helps…either way you still have to tune it! go by any current seat measurements and you will be fine. if it is the stiffer chassis you will be fast when the track is green or slick…but you will be over stuck when the track gets grippy. i learned this the hard way back in the day! lol with the extra power of the 125 you should be fine and it shouldnt be as noticable…i ran an 80cc on this same chassis and it was too stiff.