Once you’ve had your bike fully de-restricted, an Aprilia rs 125 big bore kit is usually the next thing people want to install. You can only get so much power from an engine while keeping the power in a useable rev range so increasing the displacement is really the only option if you want to increase power and torque (which translate into speed and acceleration).
Its often said that there is ‘no replacement for displacement’ and a big bore kit simple makes your engine bigger. This allows more fuel and air to be burnt within the engine on each power stroke which gives you more torque (turning force on the crank). Power is a function of torque multiplied by engine speed so if you can produce a bigger torque in your new engine whilst allowing it to rev just as easily, you can squeeze a little more power from your bike.
These power and torque increases do not come for free. The only Aprilia RS 125 big bore kit currently available gives you an extra 25cc (turning your bike into an Aprilia RS 150). This doesn’t mean your going to outpace any 600cc bikes and the kit does not come cheap. Expect to pay at least £400.
To keep your bike legal, you’ll need to re-register it and reinsure it (insurance companies do not like big bore kits or riders who install big bore kits to their bikes…expect the worst). Maintenance costs will increase as your bike will require exotic parts for engine rebuilds and as your bike is no longer stock, its resale value will decrease.
Big bore kits can be sourced online and occasionally for a bit cheaper than usual on eBay. The kit is basically a new top end for your bike so if you have rebuilt it then fitting the big bore kit is a straightforward process.
If you absolutely must fit a big bore kit to your bike, I suggest you bore out the engine and run it until it needs a rebuild. Rebuild the engine as a 125 and sell the bike on moving onto something bigger and better. You’ll get to run your bike as an Aprilia RS 154 and enjoy the extra power and torque from your bigger engine, but you shouldn’t lose too much money when you resell.