Putting LED near incadescent light source on lava lamp
The lava effect in the lava lamp is a kind of convection. It is driven by the heat of the incandescent lamp. You will need to replace the incandescent lamp with some kind of 25W heater that doesn't emit light in the first place.
While you are at it, you may want to add some ambient temperature correction for the heater power as most lava lamps don't work well when it is too hot or too cold around.
Having done that, diodes are easy. Most lava lamps work around 60C and most LEDs are pretty much OK at 60C as long as you drive them at 10-30% of their rated power. In general, lava lamps don't push the technology limits of the light intensity, so you should be pretty much OK with that.
(Why I think so: a lot of LEDs are rated for 80-120C crystal temperature, e.g. see a random LED datasheet here)
You can as well use some &quot;fiber optics&quot; (the &quot;fiber&quot; part may be 2-5mm diameter glass or transparent plastic sticks) and place the diodes well below the heater and carry the light where you need it. But it probably doesn't worth the hassle.