Any good tips for thorough cleaning the waterways in the engine block while in situ?

Just had to replace a core plug, and she's got a fair bt of crud inside. Currently waiting for a new hose so scraped out a fait bit and put the new plug in, otherwise I could have got a hose in.

There's quite a few proprietry 'rad flush' treatments I could use. She's not blocked - maintains temperature really well.

"Y mae dafad ddu ym mhob praidd"
1972 4/4, 1981 MGBGT, 1984 FLHT1340, 1980 XLH1000, 1990 ZX10