Strangely enough the very long Briggs & Stratton number is not a serial number - it tells you the year, month and day it was built, but nothing unique to that specific engine. Your engine is a model 60102, type 030301, possibly made on 12 May 1970, but you seem to have one too few digits in the last block (there are usually 8, not 7) so I can't reliably guess where one is missing from. The 60102 is a very long-running model, it has been going on forever. It is 6 cubic inches displacement (about 99 cc). The oil seal you are not happy with is in the timing cover. Replacing it should be fairly straightforward - so much so that it isn't even mentioned in the B&S overhaul manual, as far as I can tell. You would need to remove the clutch, clean the exposed part of the crankshaft, remove the screws that hold the timing cover, and slide the timing cover off the crankshaft. You then press or drive the seal out of the cover and press or drive in a new one. Putting the cover back on is the nearest thing to a tricky part: you mustn't damage the sealing lip on the inside of the seal, so you coat it with something like grease after you have polished the crankshaft to a high lustre and cleaned and oiled it, or you use a very thin shim between the seal lip and the crankshaft, then slide the shim out when the cover is back in place. Remember, to pull the seal off the shaft you are going in the easy direction, but to put it on the shaft the seal will be fighting you, which can easily damage the lip. However the joint in a wrapped shim can damage it even more easily. Any tiny nick in the new lip is likely to leave you with a worse seal than you have now.