The volume of oil/air returned by the scavenge pump should not be underestimated.......This applies to the stock pump but even more so with a 2 start pump.....If the oil tank does not breathe well enough, then oil can and will spew from the filler cap........the returning oil within the filler neck is too close to the cap.......this volume needs to be vented......I'm sure on the twin racer I ran years ago, I used a small brass fitting taped into a plain filler cap and ran a hose up forward and down, not dissimilar to a fuel tank breather line. I know it is common among some owners on here to blank off the chain oiler T 29 fitting, but to me this is a mistake.......its main purpose is a tank breather and secondly to lube the chain, if one wants to do so........The ET183 seals in the timing cover should be installed and checked one at a time......this way, you can press the cover on with the dowels centralising the cover......be mindful about whether you are using a stock cover gasket, a cometic one or no gasket at all, as this will dictate the installed height of the cover.......You want the cover to show not more than a 1mm gap at the top, adjusting the amount is easy enough by trimming down the height of the Simmons nuts on each spindle, or if the gap is too small, install a 5/16" plain washer under said nut........carry this out on all 4 seals and with the cover screws tightened, the crush on the rubber seals will not be excessive.......All too common to remove a cover and the seals are crushed way too much with the rubber deformed badly.......remember there is near nil oil pressure, so a crush of 20 to 40 thou (0.5 to 1.0 mm) is plenty.