Long term storage damage

The problems are related to the oil migrating off of the internal parts and possible local corrosion.
Your success will have a lot to do with your particular situation.
If it has been In a Dry/Heated hanger, the prognoses will be better than if it has spent the years in a humid hanger near the ocean.

If you already own it, get some pressurized Oil into it before you turn it.
If you are considering buying it, discount it deeply.

Economical?  Are you doing the work or contracting it out?

The Official Rotax Opinion is that after 24 months it is a goner.
Your Call!

I am going through that now with one . Fresh rebuild and stored for years. after about 30 hours oil leaking everywhere. . We replaced the prop seal then the main seal and stopped most of it. Next will be orings in the cylinder bolts. It  leaks or uses about about a quart every 6 hours or so now,

Might want to plan on replacing seals especially if it has not be properly been cared for like rotax recommends . Just my experience talking. 

Fred Northup wrote:

I am going through that now with one . Fresh rebuild and stored for years. after about 30 hours oil leaking everywhere. . We replaced the prop seal then the main seal and stopped most of it. Next will be orings in the cylinder bolts. It  leaks or uses about about a quart every 6 hours or so now,

Might want to plan on replacing seals especially if it has not be properly been cared for like rotax recommends . Just my experience talking. 

That sounds like a giant headache to me.

The main issue from long term storage and no run time are seals and "O" rings drying and cracking. There are "O" rings everywhere in the engine. These are the items that need replacement plus internal parts lack any oil on them and you don't want to try a start with dry metal parts.

You don't say how long it has been sitting.
3-4 years and you might have a chance.
13-14 years, No way!

I suspect you are going to give it a try in either event.  My Advice, which I am sure is going to get flak!  is ...

Get it mounted on a stand WITHOUT turning the crank.  No Prop Yet!
Pull the Oil Filter and pump Oil into the center Oil Filter mount thread.  5-10 psi should be plenty.
Expect a good deal to Dribble out the Outer Oil Filter channels.
Keep up whatever pressure you can get until you see Oil dripping from the #1 Cylinder Valve Rocker Arms.
This is the farthest point in the Oil Circuit, the engine now has Oil everywhere.

Fill the Oil Filter and re-install it.
Mount a Prop.
Install and fill the Oil Tank and PURGE the Lines.
Temporarily replace the Oil Pressure sender with a Mechanical Pressure Gauge.
HAND Crank the Prop until you see 20 psi in the engine.
BURP the Engine.
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For those who purchased an engine too soon in the build cycle and want to give it a fighting chance of surviving a couple more years ...
Get the engine mounted on a stand with the Oil tank installed and filled.
Temporarily replace the Oil Pressure sender with a mechanical gauge.
Every month or two, HAND crank the engine until you see oil pressure. 
This will keep the parts Oiled and moving.