Bike for the kid, RM85 vs KX100?

I'm looking for a trail worthy MX bike for my 12 year old. Currently he is riding a CR85 that's been bored out to a 100. The bottom end power is too flat and the power deliver too abrupt for the riding we do. I'd like to hear from parents with experience with a RM85 and KX100. My understanding is that the RM has good bottom end power and can be ridden effectively in the lower part of the power range. My understanding is that the same is true of the KX100. Can anyone shed some light on the subject?

Ideally I'd just test ride both and come to my own opinion but there are none available locally. I'll have to commit to a couple hour drive just to try one.

Talked to a few people about the 150R. I've been warned that they can be hard to start after being stalled/dropped. I had a line on one with a Rekluse, which would all but cure the stalling problem, but it came and went before I could get my hands on it. I looked into a RMZ/KLX 125. Out of the CRF, TTR and RMZ/KLX 125s they appear to be the easiest to modify. The motor is out of a 200cc duel sport bike so increasing the displacement should be straightforward. RM and KX forks are a direct bolt on and a KX swing arm will bolt on after you enlarge the mounting holes and make a couple simple spacers. The KX shock is a direct bolt on as well. Problem is you end up with a fairly well suspended bike that is still heavy . . . . . top heavy at that. I can't help but think a light, well suspended 85 or 100 with a power-valved engine is the way to go. I'd love to hear someone tell me I'm mistaken though. I have little experience with kids play bikes so maybe a TTR or similar would be a better trail bike for a semi aggressive 12 year old?

I've added steal clutch plates to increase the flywheel affect, dialed the jetting, installed a Fatty pipe and added a couple teeth on the back but its still not where it needs to be.

I know the FFW and reeds would smoothen the power, and I am tempted to give them a try before going to a different bike, but watching my son ride he really needs more bottom end as he likes to lug the engine below the power band then open it up once the trail widens out. From my experience with bigger bikes, I don’t think the results would be dramatic enough for what I’m hoping for. Developing that riding style is my fault, that's the way I ride and he is trying to emulate what I do. Maybe a better idea would be for him to keep the CR and just buy myself a 125? :)

I have a CR rolling chassis as well so I've thrown around the idea of doing a CRF100/CR hybrid.