You can apply the relevant costs of the car against your taxable income.
I dont know what the rates are but it is far more complicated than just a 15%.
If you have $40,000 in taxable income and say the tax rate at that earnings level is 25% or $10,000 you can apply expenses against that taxable income but you cannot ever get back more than the tax you pay in. in this example $10,000.
So…
If you paid $10,000 in tax this year and you use your car for work you could apply the $5000 you paid for the SR against your taxable income and in so doing you would reduce the tax that you have to pay to the GOV.
Because you have already paid that tax when your employer gives you your cheque the government will return it to you.
Now, just because the costs of the swap were $5000 doesnt mean you can claim all of that. for things like capital gains, meals and entertainment, club dues, etc. etc. you can only expense 50%. These are all rule based and I am not sure what the rule is for automotive.
Assuming that 100% of the automotive expenses can be applied against taxable income you could claim, with receipts, your $5000 engine swap. The government would then refund you $5000 of the tax that you have already paid to them.
This is the same thing as using your university or college tuition as tax credits.
For most of us who went to school you will have low earning and high costs… ie. $15,000 in earnings at the lowest tax bracket meaning you paid like $2000 in taxes.
You expense your school fees of $10000 per year against your taxable income in the form of tax credits.
You cannot expense yourself into a negative taxable income so you apply the first $2000 of tuition as tax credits and you carry forward the other $8000 into future periods.
The idea is that in a couple years you will be in a higher earnings and tax bracket with fewer expenses that can be applied as tax credits.
This is exactly the reason why myself and other university or college grads do their taxes for the first few years after school and get big fat tax returns of like $1500 - $3500… we are getting all of our taxes back because of the amount of tuition we paid in prior years.
As an example I have about $20,000 in tax credits right now due to school… assuming that I pay taxes equal to $4000 this year and every year from now on I would be elegible for upwards of a $4000 tax return each year for 5 years.
Of course it doesn’t actually work 100% that way but that is the logic behind it.