I took my motorcycle to the mechanic to repair (it) last week. I took my motorcycle to the mechanic to repair (it) last week.

I took my motorcycle to the mechanic tofor repairs last week. I took my motorcycle to the mechanic for repairs last week.

I took my motorcycle to the mechanic to repair last week. I took my motorcycle to the mechanic last week.

In the US, this is more how'd we say it. If we take a car to the mechanic, we assume it's for repairs.

I took my motorcycle to the mechanic to repair last week. I took my motorcycle to the mechanic last week.

It can be inferred that too took it in for repair. Good English usually avoids redundancy.

SinceAs the repair was done, so I went to pick it up this morning. As the repair was done, I went to pick it up this morning.

Since the repair was done, so I went to pick it up this morning. Since the repair was done, I went to pick it up this morning.

When you're saying "Since" here, you're committing the clause that it's contained within to be dependent upon its `proceeding` clause. In the proceeding clause, "so I went to pick it up this morning." the word 'so' commits this sentence to be dependent on its `preceding` clause. In essence, it reads like: I went to pick it up because -> the repair was done because -> I went to pick it up because -> the repair was done...

S(And) since the repair wasy were now done, so I went to pick it up this morning. (And) since they were now done, I went to pick it up this morning.

Since tThey finished repair was done,ing it so I went to pick it up this morning. They finished repairing it so I went to pick it up this morning.

Since the repair was done, so I went to pick it up this morning. Since the repair was done, I went to pick it up this morning.

Only use one conjunction. You already used "since," so you don't need "so."

They changed the starter motor and the relay., They changed the starter motor and the relay,

This grammar is already perfect but lets try and improve things anyways. How about we turn this simple sentence into a complex one by combining it with the next.

Iit cost about 68,000 yen. it cost about 68,000 yen.

Now that this sentence is more complex, we have to convert this fragment from it's original tense of `past simple` to

It takes a lot of money to keep an old motorcycle. It takes a lot of money to keep an old motorcycle.

Another perfect sentence, no errors. You might consider convert "keep" to the phrasal verb "keep up" to make it sound more folksy or down to earth. IE: "It takes a lot of money to keep *up* an old motorcycle."

It takes a lot of money to (keep/ maintain) an old motorcycle. It takes a lot of money to (keep/ maintain) an old motorcycle.

It takes a lot of money to keep an old motorcycle. It takes a lot of money to keep an old motorcycle.

Stylistically, this would be better at the end, as it's a nice punchline.

When I started the engine, I was surprised at how powerful the starter motor was. When I started the engine, I was surprised at how powerful the starter motor was.

Stylistically, the paragraph would flow better if this sentence was right after "They changed..." because this sentence is thematically connected. Also, that way, you can have your funny punchline at the end!