Put a new engine in a 1998 isuzu rodeo now it won't start. I thought it was the neutral safety switch because the R was on the dash and it was in park

This could be the problem. If your dash indicator is showing reverse but the transmission is in park it could be the safety switch. I'm not talking about the safety switch in the transmission though. In automatics they put a second switch to kill the starter from engaging. You need to adjust the linkage. If your shifter isn't actually on the column but the indicators are there is a cable that attaches the floor shift to the column and that is adjustable. Put the Floor shifter in Park and turn the column section nearest the dash by hand until you find park. Then adjust the cable length as needed. If all you have is the column shift it is still a cable adjustment. Set the emergency brake, drop the shifter to low 1 and adjust the cable linkage by loosening it on the transmission and manually move it all the way to low 1 then tighten it down then move the shifter to park and repeat teh adjustment process to verify that it is properly aligned. Lock the linkage back down and see if the vehicle will start, if it doesn't move the shifter to neutral and try to start the vehicle if the vehicle starts then check the column shift safety switch at the park position to make sure it is still in place and working. Typically, if the vehicle won't start in Park or Neutral it is more likely to be the Transmission neutral safety switch.