Need advice

Hi all new to this forum. I'm seeking some advice from others. Back story is I rode from like 7 years old to 18 and got into so trouble had to sell the bike. Fast forwards I just got a 2019 kx450 and I got my son a 1999 jr50 and my daughter a 2001 xr70 we ride trails alot and the 70 has been perfect for my daughter to learn on and the 50 is great in the yard but doesn't have the power to go up any hills and in any mud with out just stopping. So far I have moded a z50 rear sprocket 35toith to the rear and a 11 front (stock is 13 28). We went to a flat track race today for my son's first time and he just didn't have the power to really compete there was 2 husky tc50s and they were power houses. So I'm now kinda considering getting him something like a ktm 50 Jr or the husky. I think I might just need some one to convince me it's a terrible idea. My son is 3 and can rip pretty good he has been on the bike for say 4 months and can go through the tight trails all the way to the power lines whick is prob a half mile with roots and hills and stuff and he loves it but he really liked racing too I'm unsure if I should just let him be happy with just being able to ride in the race and do pretty good in the trails or if I should try and get him a more powerful bike. Sorry for the long winded post. I didn't write it very clear but with the new gears his bike does pretty decent in the trails but with the taller gear doesn't have the power to do anything unless he's really moving