My bike was previously a 26er mtb 3x7speed and I converted it now to be a single speed freewheel bike.
Is the bike safe to use if the rear wheel is not straight to the frame? It is closer to the drive side. The cog's and crank's chainline are straight with each other but the wheel is not at the center of the frame; I tried to ride it and it seems fine but I'm wondering if I should let it be just like that?
It appears you did something along the lines of take a multi-speed freewheel hub, put a BMX freewheel on it, then rearrange the spacers to get the chainline dialed with the front.
Having the rear wheel offset from the frame centerline will disrupt handling and it is possible to imagine safety repercussions on that basis. Many would find it less pleasant to ride.
The simple answer that doesn't involve buying anything further is to re-dish the wheel. Presuming a wheel that was dished right before, you always do that when moving axle hardware around for any purpose.
In a proper singlespeed conversion, the hub is reconfigured to center the flanges on the frame centerline for a dishless wheel, and the cranks/BB/chainring are reconfigured to match the resultant rear chainline. That way you're not throwing away one of the best things about singlespeeds, the dishless rear end.
