Not specifically trials related but im hoping someone can help.

I am thinking of modifying my klr 650, puting on a custom subframe and exhaust ect. 

For a younger rider does anyone have an idea how much of an effect it may have on insurance?

Cheers    

As the subframe is not the main frame, you may find this is not regarded as a change, the same as lowering kits etc.

the exhaust, if it's for more power etc then that could be expensive and the insurance company may want a Dyno report as the bike may move insurance groups. If it's not to increase power, just cause the old ones knackered and you like the sound of the new one, let them know but it may not be an increase.

These opinions are based on some of my experiences. Some insurance companies will give you advice before you make your changes. May be worth a phone call.