If your GPR50 is getting tired or you just want some more power, fitting a big bore kit might be the way to go. Many owners fit a big bore or engine tuning kit when the original motor needs a rebuild and the cost of a 70cc kit is often a similar price to buying new parts.
If you are handy with the spanners, fitting a new piston and barrel is fairly straightforward but it’s not a 5-minute job. If you are in any way unsure of your capabilities, it’s a job which is probably best left to a qualified mechanic. Many 2-stroke engine rebuild kits are more complete and come with new crankshaft and bearings, but fitting will require a full strip down.
Before you buy a big bore kit for your Derbi GPR50 remember that you MUST tell your insurance company before you ride it. If you don’t you will invalidate your insurance.