The caps are not air tight. Making them would stop the fuel from going to the carb. You can take an inner tube and cut out the shape of the top and place it in the cap on top of the existing gasket. It's still not air tight but the splashing will be limited.
I used heavy duty washers on both sides of the chain tensioner. I also swithed out the standard wheel to a bearing wheel to make sure I don't melt the wheel at high speeds. I also changed out the rear sprocket assembly to an adaptor hub from LIVE FAST MOTORS
. Their spring loaded chain tensioner also works well. Check the alignment between the drive sprocket and rear sprocket. Also check the tension on the chain, you may be able to shorten the chain. The amount of riding you have done may have seated the motor a little more and chain may have stretched.