5hp Briggs & Stratton

[Alanky]

I have a 5 hp briggs engine (same one on kirby's new reel mower only with a racing carb and stright pipe muffler) i had not run it for awhile so i started it and was letting it idle. It let out a VERY loud backfire and shot fire, smoke, and some chunks of stuff out of the muffler. Now it wont even try to start. Replaced some gaskets and small parts and it will run poorly for about 3 seconds and die. When i pulled the recoil cover off there was oil all over the head and block so i wiped it all off and turned the flywheel with my hand and noticed oil bubbling out, but cant see a crack or any thing and i know oil shouldnt be coming out! so what do you all think could be my problem here?? small crack in the block i guess but aything else? I have a cast iron block that i plan on putting all my parts into when i get time. it has also lost a little less than half its compression. Any help is welcome!! Thanks in advance
Alan