Just bought a xr250 cant start not sure of technique,can an electric start be fitted

Stator/magneto failing. takes more juice to rev the bike, so it takes from the lighting.

If you don't have a battery the lights will go off when the engine stops. If you have removed the battery you will kill the electrical system ( Very Expensive)

Alternator ? Charging system problem ? Do you know how to do automotive electrical testing ? Because that is what it's going to take to find the problem ! Your best bet take it to a qualified repair shop ! Or learn how to do electrical trouble shooting ! Do you know what a wiring diagram is ? how to use one to find point's in a electrical circuit to test at ?
Electric Testing Techniques You Need to Know
Motor Age TST New Battery Starting Charging System Testing Techniques
Did you check for blown fuse's ? Blown fuse's happen because of shorts .
Finding The Source Of Electrical Shorts In Your Car

Basic Electricity for Service Techs Ohm law Current Flow Opens Shorts

take a long crowbar fit it between the bolts and crack the bolts with a breaker bar in an opposing or star pattern use a mapp gas or propane torch VERY CARFULY hit the area around the bold for 4 seconds to overcome any loctite if you cant break with a bar by hand first there is a better way but it involves a specialty tool or two

there are fuses located on the gen..check those

The kick shaft gear, idler gear, and clutch drum gear on the 1985 xr250 and below are smaller tooth gears than 1986 and newer. I have an 85 and an 88. I had to replace the 85 kick gears 3 times. The last time I replaced them I found that the larger gears on the 88 fit as long as you replace only the gears. The kick gear comes off the shaft and the larger tooth gear fits on the shaft. The larger idler gear fits where the smaller tooth idler was and the drum gear comes out and the larger tooth gear will fit there. Or the drum you have with the larger gear will now fit the other gears. This is a better solution than finding another drum with the smaller tooth gear because the smaller tooth gears will not last. I hope this helps.

It is possible that your bike has been governed from the factory to only reach a certain maximum RPM, they do this so you don't inadvertently over rev the engine and blow it up.

If the bike is not governed you might have to re jet the carb, altitude and other factors can change your fuel/air requirements and cause improper operation of your fuel system therefore the maximum RPM.

Check your spark plug. If it's black (rich mixture), replace it. (There may be carb/ignition/air intake issues causing the rich mixture.) But it's probably your starting technique. I suggest getting an owner's manual or contacting a dealer who can instruct you in the proper starting procedure.