Can you get a stiffer rear shock for an 80?

I have a KX80 and YZ80 that me and my buddies love to beat around on. Only problem is we are way to big for these bikes. The rear shocks just cant handle the weight. I know I should be riding a 125 or 250....but these little 80s are just too much fun. So small yet fast. Are there any aftermarket options? Maybe a 125 shock? Any tips are greatly appreciated.

Thanks