Guys i have got a electric rc car 1:10 size which i want to covert to petrol
I already have an engine  and fuel tank
But i need help for what parts will i require to convert it
Thank you

What engine you are using ... You just need to see the correct positioning of the engine along with the gears it has to meshed with so that it can transfer power ...
Details on this like what you using May help to guide better

Well i had bought a random engine and put it on my inline skates
Now i dont skate any more and have a 30cc petrol engine lying around useless
I had connected it to the skates with a belt  drive system
Dont have any details about it but will post some pictures soon

As far as I can imagine the engine size would be close to the engines which BAJA has ...
There would be difficulty in mounting it on 1/10 chassis
You need to fabricate chassis according to the size of the engine .. Have to work around the engine ... You can use shocks tires diff as same but that will depend on the weight to power ratio ...
Take an aluminum sheet put engine in the centre along with the fuel tank mount it and then design your RC ... Wether it be a rear wheel as BAJA or 4 wd ..

You can also look on YouTube as reference ... Custom made RC ..
It would be more of help

The system is already 4wd so i guess i would keep it 4wd

I would say, it's really not the effort. If it's just a project, you can try. The 1/10 Scale drive train will get busted in no time. Try it on a 1/8, maybe with creative engine mounting and clutch design you can succeed.