I figure you have all ready pulled the motor or what ever... but if not here are some ideas.
On your EA-81 flywheel you would have to determine 0-deg timing mark and the center dash mark for the Timing belt mark, while flywheel is in place and all through a 2x3" view port. Possible???
If you had an EA-82 flywheel that you could work with and could determine the relationship (degrees) between the o-timing mark (TDC) to the degree of angle to the center dash (timing belt alignment mark) of the three dashes this info can be used on your EA-81 flywheel. Set the ea-82 flywheel on a large piece of paper, draw an outline, mark the o-deg timing mark and center dash mark tothe paper, figure out the exact center of the circle, draw a line from your timing mark to center and center dash mark to center on paper. This is the degree of angle between the two. Lets say 37.5 degrees counter clockwise from 0-timing mark gives you one mark. Now make a circle of the ea-81 flywheel (I believe the ea-81 is small o.d. so you will have to know the radius or circumferance, so make that circle and or calculate out the small o.d. of travel to equal the circumferance of the 37.5 deg from the 0-timing mark. Your degree won't change, but your distance from zero will as your fly wheel I believe has changed size so from 0 to center dash mark may have been 9" on your ea-82, the distance on an ea-81 mayb be 8.35 inches and that distance will have to be transfered to the installed flywheel.
Now you would have to determin what your zero degree timing mark is on your installed EA-81 flywheel as your currnet marks mean nothing. By positioning your engine to where the engine is exactly at TDC so it will be firing on the number one cylinder that will give you 0-deg for timing mark, mark that with white paint, your B.T.D.C. (Before top dead center) marks for running can be marked later. Now you would have to rotate the engine (flywheel o.d.) as specific distance to the timing belt mark.
To put an engine at the exact TDC at 0 deg will be an approximate, possibly at the timing marks any slight movement of the main crank could play out at + or - an inch or two at the fly wheel? and if you set the timing mark off an inch or two then that will set the Timing Belt mark off as you are using your TDC mark as a reference point. So on the number one cylinder you would have to come up with a way to determine T.D.C. of number one piston on the compression stroke and get it very close. Possibly slide a stick to the head on the compression stroke and see if you can move the engine where the stick stops moving at the top of the compression storke, note possibly the key slot on the main cranke, probably no visible difference but that may give you a reference, find the center of the slopp as you slowly pull the motor through the top of the compression stroke to where the piston begins its down stroke, find the center slopp betwee that movement and call that center....tricky and sloppy at best.. and how accurate is that to the actual 0-degree, 1/4" off is probably a tooth on the timing belt which means it may not fire if the timing itself is off...
So, find o-deg, TDC Top Dead Center on number one piston. Mark flywheel. Rotate a specific distance to that determined by the layout and make your timing belt mark. put timing belts on to that mark and turn the key. doesn't fire, move belts one tooth one way, no fire, move belts back one tooth past the original the other... starts to fire.. your getting close...
General Disorder has given you the best advice and probably rolling eyes at this reply but I am just sitting back thinking... how could a person do this if he had too... and just tossing out ideas. Once you have it running you could take a center punch and make your own marks permenant and there you go...