Welcome to the site !!
First question is
Have you contacted the shop that did the work ??
Second question is
Have you confirmed it is the valves ?? , or is this just speculation based on what it did ??
If they are reputable they should stand behind it unless you were out there hitting the rev limiter or riding around real hard on a fresh build without breaking it in properly
Did they assemble the engine , and/or did they just replace the piston/rings , or did they do the valves as well ?? , or a complete engine rebuild (crank piston,valves,etc) , did you do any of the work ?? , if so what specifically , and what is your level of mechanical skills (not judging , it just helps in knowing what you were able to confidently do , or would be able to repair if needed)
More specifics would help , as its hard to give a helpful answer without knowing the details of what was done (with the rebuild and by whom) , and what you were doing when it died , and did it just die , but otherwise the engine turns over but just wont run ?? , or is there a mechanical noise when trying to start it , ?? , have you determined there is spark ?? , did it sit for more than a month ?? , and if so did you drain the old gas and replace it with FRESH premium ?? , more specifics are needed here