I have a HPI nitro MT RS4 with a 15fe with the chrome head. (bought in 2000, it sat idle for 2003-2009 and again 2010 to this spring)
I recently stripped the glow plug threads in the head and spent a lot of time looking for info for a replacement. (discontinued, so no direct parts available)
I figured out that the T-15, T-15L, and T3.0 were all listed as replacements for each other back to the 15FE with the purple head, so I went ahead and got the T3.0 for my car. Non-HPI engines cost more and, at best, require mounting blocks so I did not branch off and try another brand.
It turns out the flywheel is keyed on the 15fe with chrome head and would not work on the T3.0. Based on pictures I found the 15fe with purple head is not keyed and that flywheel may slide on with no problems. The threaded drive shaft was a bit longer (about 1mm) as well. I had to get a new flywheel
Tower hobbies part # LXBTA6
and also got a new clutch pad/spring set
Tower hobbies part # LXCFR3
because I figured that would be an issue as well. Two small washers I had sitting around worked to fill the space between the flywheel and the pilot shaft and my car is finally working again. I have no idea if these were the best parts to choose in this case since I could not find any good info. (My post to this forum needed approval by the moderator and I never saw it show up, HPI support never got back to me, tower hobby support felt the motor/model was to old to make statements about what would work for sure)
I was able to reuse a tuned header/pipe and the stock header looks like it would fit in the same manner. I also reused my clutch bell, needle bearing, and pilot shaft.
Performance wise, I just run around the back yard and the increase in power is very obvious. Running half throttle gets more speed than the 15fe did at full speed and I don't have the space/terrain (short grass/packed gravel driveway) to try full throttle at this point.
I had HPI racing CVs (They look like LXCJGF) on the car and the stronger engine kept breaking the wire/pins connecting the joint in the rear axle. I put the stock dogbones and axles back in the rear and ran 2 two tanks of fuel through it with no problems since that change. I expect more things to break down the road since the T3 has a lot more power than the 15fe but for now I can finally claim success. It may have been smarter to get a T-15 or T-15L durability wise (half the HP of the T3) but they cost the same and they appear to be discontinued as well.
Anyway, I could not find any info on this when searching so I figured I would add my experiences to this thread in case someone cares about it later. It's the most recent 15fe thread I found.