An engine does not travel in the normal post. It ships crated on a pallet as less-than-truckload freight, and that works differently from a parcel in several respects. LTL means your pallet shares a trailer with other freight, which makes it affordable but adds handling and transit time. Delivery is usually kerbside, meaning think about how the pallet comes off the truck – a liftgate or loading equipment is often required. Check the pallet before you sign. This matters more than anything else: recording any damage at the point of delivery is what makes a freight claim possible. Accepting it as undamaged then finding a problem later makes recovery difficult. Insurance on the shipment is worth having given the weight and value. Combined with fitment verification before dispatch, that addresses the two things that genuinely cause problems when buying powertrain online.
Horsepower is the first number people look at, but transom height is what catches people out. Measure the transom first: a 15 inch shaft on a 20 inch transom will not push the boat properly no matter how much power you bought. Next work out how the motor will be controlled. Tiller steering suits small boats and dinghies, while a console setup requires cables, a binnacle and a harness which is easy to forget in the budget. Two stroke or four stroke matters less than it used to though modern four strokes burn less fuel and run quieter at the cost of weight. On a small boat that weight affects trim, so look up the transom rating for your hull. Last, ordering a powertrain sight unseen is a real commitment, so checking the fit before dispatch plus a startup warranty matter more than saving a few dollars.
Any engine change is half the project: the transmission has to match the power as well as the application. The bolt pattern comes first, but capacity is what fails if ignored. The 4L60E on a stock LS works well, whereas the same unit with significant power added becomes the weak link. Next there is the electronics. An electronically controlled automatic needs the right control strategy, plus the correct speed sensor input; without that the shifts will be poor or absent. Packaging is the other half: the transmission tunnel, crossmember position, the prop shaft plus shifter location all change when the gearbox changes. Check clearances first. Buying engine and transmission together reduces the matching problem, particularly if fitment is checked before the pallet ships.
The term JDM stands for Japanese Domestic Market, which describes units pulled from vehicles sold in the Japanese market and not export models. The appeal comes down to comparatively low mileage plus variants that were never sold elsewhere. The mileage advantage is real but has a cause: the shaken inspection regime made running high-mileage cars expensive, with the result that cars are retired earlier. Due diligence: a compression test, that it rotates by hand, which accessories come with it, plus whether the wiring and ECU are included. Differences in emissions equipment can complicate compliance depending on where you are. A JDM engine often represent real value, especially in a project car where factory specification does not matter. Approach it as a used engine with good odds and not a new one.
A rebuild appears cheaper at first glance, and sometimes it is. A fair comparison has to include machine shop time, parts, plus the risk that the block or crank is beyond saving after inspection. Buying a crate engine trades the unknowns for a known price. You also get a warranty plus documented internals rather than a history you cannot verify. For common platforms such as the LS family or the 5.0L Coyote, crate availability is good so the economics usually favour the crate. Rebuilding still makes sense is on unusual or numbers-matching Aero Engines, for which no crate option exists or keeping the original block has value. One practical point: a crate engine often does not include everything, meaning read the specification carefully – accessories, harness, ECU and flywheel may be separate purchases.