Pipes/collector

Hi I just picked up an 82 Seca for my old lady, The guy gave it to me because it was leaking oil, After various tests including compression I figured out that because the oil warning light was on most of the time he kept adding oil, There was 7 litres of oil in it and it was blowing back through the air breather, Change oil and filter but still occasional oil light warning and battery warning, I havent got that far yet lol.
Anyway the mufflers are pretty much shot and also after the oil issues the collector box seems to be soaked with oil as its smoking really bad, I thought it would burn off prety quick but not so. I.m going to take the mufflers and collector box off to see if theres a leak under the crank case dropping onto the collector. Has anyone rigged anything to replace the collector and then be able to use aftermarket slip ons? I know Mac make a 4 into 1 system but if theres a cheaper way id prefer that route, and any tips on the oil/ battery warning signals would be appreciated.
Thanks