If you can't locate a replacement eng(briggs,kohler)that you can fit/shoehorn in and eng a smoker due to ring/cyl wear consider rebuilding eng,if you decide to rebuild,disassemble eng & check/measure crank rod journal,if crank journal within specs check rod bearing & see if within specs,if not buy a new rod(oem/aftermarket)
Take the block to a machine shop that does eng boreing,have them check wear run out in cyl bore,IF greater than .020" buy a std size piston set(oem/aftermarket)for an HH120 eng(12hp)and have block bored for new piston set,I've done this with two HH100 eng as the cyl bore was over .020" out of round,the .030"/.040" piston sets have been nla for almost 10 yrs,if you need a new rod don't buy same till piston set/rebore resolved,depending on parts needed,where you buy parts & cost of rebore & gasket set cost could run $150-$300 to rebuild.
Another issue might be alt for charging battery,many eng today only have a single wire output from alt & you have to buy a new regulator or regulator is mounted on eng & you have to disconnect old reg & connect alt output to wire that connected to center pin on old rectifer/regulator,most charging systems on todays eng will not work with existing reg.
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