740W/HP. For example, a 20hp engine could be replaced with a 15kw motor (make sure that is the continuous rating). You need to make sure you gear it correctly though which shouldn't be too hard with a belt. The trick with a belt is that you need to add a bearing to the shaft.
The weight in the system primarily is in the batteries. I would guess that you can get a motor and mounts to be 75lbs and another 75lbs for the charger, controller, contactors, and wiring. Having it be lighter than the diesel is probably not realistic if you want it to propel you anywhere.
Figure out how much power you expect to use for how long and you can get the KWH requirement for the batteries. Then you can look at batteries and figure out what you need/what weight. You will have to get a battery pack that allows you to configure it so that the voltage is compatible with the motor (usually 48,72,120,or 156V, higher is better for efficiency and smaller battery cables). There was another thread here that touched on this subject and I did a very basic calculation for a 15kw motor run at WOT for 2 hours and found you would need 18 Trojan T-105 batteries(I hope I am remembering that right). Now if you throttled back to 5kw, you would only need 1/3 the battery capacity. Lead acid batteries are not great for energy/weight but they are often used since they are the cheapest. If you went with 8 of these batteries in series, you could use 48V components and you would be able to get in and out of the slip just fine but you would have a few hundred pounds tied up in batteries. This combo would probably get you home 10 miles in flat calm conditions on a 30 footer as well.