I still recall my first mod on a small motocross 50cc 2 stroke motorcycle I had when I was like 15. The thing had 6 gears, but due to my weight paired by the heavily underpowered engine, I barely could use it up the 5th gear, therefore something had to be done. That was in the early 80s, and the engine was probably a mid 70s design with cast iron cylinder. Some then friends of mine who were into modding for fun told me they could do the work if I bought the mod, so I did and a few days later we were in their garage. The mod consisted in the complete head, cylinder and piston (still 50cc but racing grade chromed aluminium cylinder etc.), plus a new carburetor (from 14/9 to 22 iirc). After the mod was done it took a while to turn it on for the first time, but then after some tuning it became reliable and I could get back home. I spent a couple days driving it, then the engine seized. Having done anything wrong, I called those friends to ask explanations, but as predicted they weren't collaborative, so I bought a second mod (reforming the chromed aluminium cylinder would cost more back then) and this time mounted it by myself. Turns out the old one broke because one of the G-clips on the piston was not properly secured and fell out of its receptacle destroying the cylinder chamber. The new mod, although performed by a complete novice, lasted some good years until the day some bastard stole my motorcycle.

In the end, it cost me all the money I had saved, but was a fun and instructional experience nonetheless, also for teaching me that some friends when things go wrong often stop being such.