can someone tell me if i can replace a sears gt 12hp with a kohler engine 12 hp the sears is a techumseh engine and is bad a engine on my front end loader tractor and is needed to plow snow as it lifts up and has down pressure too,also the kohler is the generator/starter type and runs like new has been totally rebuilt thanks for any info i can find am about to start this project

Engine "repowering" is a sticky subject, perhaps a bit too in depth for this forum but I can offer some advice...
Areas of concern:
-Overall dimension of engine. Dipstick tube, air cleaner, blower housing etc. - Will hood close, will anything interfere? You said the Kohler has a starter-generator, which takes up a fair amount of room.
-Cooling air blockage by rotating screen - even in winter some cooling air needs to get in.
-Footprint and bolt pattern/spread.
-Crankshaft diameter and length, and keyway width/style. Else new pullies are needed.
-Crankshaft height.
-PTO mounts - i.e. anything bolting up by the crank area - bolt patterns etc.
-Exhaust interference, and will it point at anything? Also will it be too close to anything?
-Ignition system - old engine probably magneto, Kohler will most likely need battery power to operate, and if so will necessitate a new keyswitch and wiring up engine correctly. You tell us it has a starter-gen and is 12hp, so it probably is a K-301, or an older series. MOST K-series, like 98% had battery-powered point based ignition. A few rare K-series had solid state.
-Fuel delivery. Many tractors have fuel pumps as tank is near or below carb. level. Need to compare engines, electrical or impulse pump can be added.
-Throttle and choke controls.
-Fuel line routing, although usually a non-issue.
-Oil drain.
-Charging system - does the Kohler have charging system; a plain rectifier, or a regulator? Rewiring again will be in order, and possibly part of the ign. switch change requirement. (Older, such as K-series) Kohler charging systems must be disconnected from battery when key is turned off, else it backfeeds and drains the battery.
-See, not as easy as it sounds. A good start is to just place the engine on the frame where the old engine was, and look things over.
Paul

thank you Paul you have listed the many things i may have over looked while doing this re-power i really think i have to make another choice, and go thru my many engines and find a direct replacment a techumseh 12 hp will go where the green 10 hp was ,here is the real problem ,, the exhaust , it just broke off the engine and has very few threads left on engine i started another piece of pipe into engine but soon vibrated off and fell to ground im wondering if i can find a brass conector of some kind and screw it in as tight as i can get it and hope it will hold up ,or i will need another block and go from there to bad this had to happen as it runs strong the green 5 star engine was rebuilt and on a snowblower when i got it for free sno thrower was shot so i kept engine but its done now i guess but still runs with no exhaust at all right off the block thanks again for the reply a very good list of info , you have done this once i can tell timbo

Timbo, you can actually buy a Heli-Coil thread repair kit for NPT - which is what the exhaust port is threaded. I used to repair B&S exhaust ports on 21" mowers years ago, but the kit will probably set you back $50 to $75.
As for a direct replacement, you probably won't find one anywhere but used, save for any leftover inventory somewhere. Tecumseh stopped making most all of their flatheads years ago.

i have decided to install a cast iron ohv 14 hp out of a craftsman (gt14hp) the engine runs very well and is out ,, ready to install,, however it has rubber mounts and i may have to drill my other tractor frame to install this engine into it? otherwise i have to rebuild another block i have 12 hp thats exactly like the one with bad exhaust port 10 hp techumseh ill need to find wireing diagram for electric start so it will/can charge the battery as for that ill use a car battery mounted to the back for weight and electric power for lights etc, i know the diagram is online but i have no idea where exactly to find it , its just the simple wireing for a garden tractor with 2 headlights selenoid ,electric starter,key switch and battery and ammeter it should be well with in my skill level just that i go from pictures or diagram to get right the first time would be really nice as my shop isnt heated at all but i work infront of a salimander heater ...ialso have a 16 hp cast ohv engine too that has been rebuilt with brand new carb on it but that runs so well on tractor its on... ill not part that out..,,, thanks for all the help i have recieved here it has been a tough struggle to fix my old stuff without all of you who know it so well timbo

well moter is in and guess what it started on first try ,complete with ameter and electric starter one good engine no smoke at all runs smooth as new and operates my front end loader like magic must be luck was with me never done it before with out knowledge of the fact the moter would fit i didnt know for sure but now i does it fit nice and simple thanks to paul44 for input on my situation hope all had a wonderfull holiday i spent mine in my shop and made out well anyway timbo