The 1995 & earlier cylinder liners are TOO Thin at the base to be safely overbore to any thing beyond 661cc. 1996 & newer cylinders (with thicker liners) will fit our OLD engines and can be safely overbored to 692cc.
The most popular overbore size for 1996 & up cylinders is 685cc, which is 2.5mm over standard & still allows one additional oversize if ever the need be.
To go over 692cc, ones needs a 1996 & up cylinder and then have an even thicker cylinder sleeve installed into it After enlarging its opening and clearancing the water jacket around it! More parts, more labor, more money, not really worth it to most people.
Due to parts access and possibly dependant upon available higher octane fuel, like 95 MON, a Wossener 11:1 compression piston kit, in 101.0mm size & proper cylinder head porting and valve seat re-conditioning and opening up the top of the air filter box may be just the ticket for more power at a reasonable price in your homeland.
Even the stock 651cc engine can make 40+ HP with proper attention to details, as seen here, (click on the graphs) https://www.klrchris.com/
(Also click on any BOLDED
items.) like air box mod
A few more mods gets a few more HP, https://www.klrchris.com/klr650-small-port-head/
klrchris had to of built his 1st set of the adjustable camshaft gears earlier in this project or for another customers bike, by the HP readings in this section.
With sprocket cores exchanged, they are quite affordable and probably a wiser & cheaper single item purchase than a loud pipe, imo. But shipping cores from Finland may not be justifiable.
Of course it would be a complete waste of time, energy & money to allow a couple parts worth less than $50 USD to destroy a rebuilt engine. Are you aware of the engine Balancer Chain adjuster & spring issues? AKA the Doo-Hickey.
Here is a complete expose-ay with videos, https://www.souperdoo.com/stuff%20th...bout/doohickey
Have FUN and looking forward to reading about your rebuild in these pages.