The RC51 allows two over-bores, .25mm and .5mm.
However, since the cylinders are integrated into the upper case half, that will entail engine disassembly.
A pain, but fortunately not frequently needed.
If the engine has already been bored to .5mm max OEM limit, you can send your case half to Millenium Technologies and have them coat the cylinders with Nikasil back to std. OEM diameter.
Or, you can scout around for someone selling a used RC51 engine in good condition.