Put together a list of every shop related to the auto world within a 100 mile radius. Including personal builders and fabricators out of their garage that do side jobs. Whether it be an aftermarket bolt on shop to machine shops to body shops to pull yards. Map them all out visit every one of them personally. Call those that your not able to visit. Dont overlook anything. Check Iinto every group whether it be motorcycle up to tractors. Go to every show or cruise and talk with everyone. Go on late night cruises and look for groups and seperate club related meetings hotels restaurants retail parking lots and speak with everyone there. Dont single every one out but be suspicious of everyone. All the while search deep within the auto world online like gearhead said. Check every sale site and every forum involving the use for such a motor. Keep an eye out for builds within your area see a project with their garage door open stop by n start a yakjaw session. Also keep an eye out for new finished/driveable builds parked or being driven around. The entire motor or parted out could be used in anything....a lawn mover a motorcycle a sport compact a low build of any kind a mudder or a crawler anything.
Youll be super lucky to find it in a whole and it hasnt been started on yet. More then likely youll only get the block but on the other hand since the entire motor was taken check personal contacts that are either building a project or have one coming up. An entire motor takes time to do anything with. Be stern and intimidating but in a manner of coming off in a nice manner.
Its hard to swipe an entire motor, whoever took it has the means to do so. Personal relations that knew of the motor and with the means to pull it off is a good starting point. I apologize for this not being very orderly but I hope it helps ya a little bit.
Do not overlook anything or anyone. It takes alot of time money and energy so if your comfortable doing so ask every friend to help you search for it as well. Even though one or some of them may be involved they will show signs and the ones that arent may give up info on the ones that did.