I know for a fact the beast wont do mods - i bought a built x0x off him and he wouldn't do it at time of build. Hes very much a purist and didnt want his name associated with anything other than the pure 303/x0x sound. He built an excellent base unit tho.
Although in Sweden, i think phono on these forums is your best bet. I dont know of any modders here in the UK, I contacted a few people about getting a ready modded x0x when i bought one and most people wouldnt mod it. phono has a great rep and haws done a lot of mods before.
Nice chap as well.
His website is www.dinsync.info
. A thread on another forum detailing the build and mod of a x0x is HERE
Otherwise a couple of people on here have mentioned Andy Pledger (search the forum) but i dont know anything about him TBH. Apparently good, but dont know if he does mods.