If an engine is optimized for ethanol, it will likely use a higher compression ratio to take advantage of ethanol's high octane (105 or 120, can't remember which) to boost efficiency. Fuel injectors (or carburetor jets) will be nearly twice as large as with gasoline. Fuel tank should probably be enlarge accordingly to maintain the same range. Cold starting WILL be a problem on pure alcohol. This is the reason for using M85 or E85, which is 15% gasoline and 85% alcohol. Due to alcohol's affinity for water, the fuel system metallurgy will probably need to be upgraded to stainless steel. Elastomers (O-rings, etc) in the fuel system may need to be changed.