im not a big fan of engine flush. it can bring up chunks and block the filter or bring up metal swarf and do more damage than its worth..

change the tappets over, but first run the engine until its luke warm, and then drain the oil ( change the filter obviously ) then do the tappets, put it all back together fill it up with oil and runn it at about 2000rpm for a minute or to.

to be honest it will sound like absolute shite to start with as the tappets will not hav built up pressure


hope this helps